What is this tool/thing in an Aztec painting?

It's cuitlatl, a symbol of excrement, sin, perhaps fire.

We see plate 57 from the Codex Borgia. It depicts Tlazolteotl goddess of earth and filth, here as a moon goddess and goddess of pulque, together with her consort Patecatl god of healing, fertility, peyote and pulque again, as a moon god. He represents a monkey, she movement. Both are the lords of the North, sitting below a half-sun half-moon disc representing the passage from day to night.

The gods in each panel are shown as married pairs. […] In the second panel the deities are Patécatl and Tlazoltéotl as the god and goddess of pulque, the intoxicating liquor made from the sweet sap of the maguey. They sit on either side of a vessel marked with a skull and two intertwined snakes. The disc of the heavens is half sun and half obscure night sky, an image that is probably appropriate to pulque.
_{–– Gisele Diaz and Alan Rodgers : with an introduction and captions by Bruce E. Byland: "The Codex Borgia: a full-color restoration of the ancient Mexican manuscript", Dover: , 1993. (online)}

According to the commentary on this by Eduard Seler the symbol to the left of the Patecatl seems to be cuitlatl sign, symbolising flame and smoke, alluding to the effect pulque gives to the drinker.

Endlich sind noch, zur Rechten und zur Linken, über den erhobenen Händen der beiden Götter, gewissermassen von ihnen dargereicht, eine Schale mit einem Jaguar und eine Schale mit drei Steinmessern, aus denen Flammen hervorbrechen, endlich unten vor dem männlichen Gotte das mit Rauchwolken versehene cuitlatl-Zeichen, das Abbild des Feuers, dargestellt, Symbole, die natürlich wiederum in durchaus verständlicher Weise die Natur des Getränkes zur Anschauung bringen.
_{–– Eduard Seler: "Der Codex Borgia. Eine altmexikanische Bilderschrift in der Bibliothek Congregatio de Propaganda Fide, Band II, Tafel 29–76", Berlin, 1906. (p173) Online at FAMSI.org. and archive.org}

The exact meaning of cuitlatl in this context currently escapes me. But Nahuatl is quite flowery in its imagery for word meanings:

The Nahualt legend of the Sun and the Moon can be partly interpreted as a myth about metallurgy. Precious metals like gold (teocuitlatl, literally ‘god’s faeces’, from teotl gods cuitlatl faeces) were strongly associated with the Sun and the Moon.
_{–– César E. Giraldo Herrera: "Microbes and Other Shamanic Beings", Palgrave Macmillan: Cham, 2018. (PDF)}

But in Selers time the meaning of excrement for cuitlatl was already established, and he associated it again with fire (and sinfulness):

Es wird dort von den Interpreten mit der "caida del primer hombre" in Zusammenhang gebracht, d. h. mit dem Märchen, auf das wir bei dem sechszehnten Zeichen noch zu sprechen kommen werden, von dem Lande Tamoanchan oder Xochitlicacan, wo Tonacatecutli und Tonacacivatl' lebten, und aus dem die Kinder dieses Urgottes, weil sie eine Blume brachen, hinausgeworfen wurden und zur Erde kamen. An Tamoanchan werden wir wohl denken müssen, aber nicht, oder nicht in erster Linie, an das Land des Sündenfalles, sondern an das Land der Geburt. Denn dass es sich auch bei diesem Gotte, wie bei seinem Zeichen, der Eidechse, um Zeugung, um einen geschlechtliehen Verkehr handelt, das scheint durch die Blutschlange, die wir in dem Bilde unserer Handschrift und auch in dem einen Bilde, Abb. 292, des Codes Vaticanus aus dem Munde dieses Thieres hervorragen sehen, und mehr noch durch das Blutband, das in dem Bilde unserer Handschrift aus dem Munde des Coyote in den Schnabel der über ihm dargestellten Vogelfigur übergeht, angedeutet zu sein. Genau ebenso sehen wir ja auf Blatt 61 des Coilex Borgia bei Tonacatecutli, dem Herrn der Zeugung, das dort dargestellte und augenscheinlich doch in Kopulation gedachte erste Menschenpaar durch ein von Mund zu Mund gehendes breites Blutband verbunden.
Denn einerseits sehen wir aus dem Munde des Coyote auch hier die Blutschlange hervorkommen. Andererseits ist die Kette von chalchiuitl-'Perlen, die das Thier in den Pfoten hält, in einen Stab umgewandelt, der mit einem Thierkopfe in den Boden beisst, und zugleich von gelben Streifen eingefasst, die man als Feuer oder, wenn man will, als cuitlatl, als Exkremente, Unrath, den Ausdruck der Sünde, auffassen kann.
_{(–– Seler, p99)}

cuitlatl, Exkremente, Schmutz = Sünde (s. tlaelquani). I 39, 94, 99, 123, 128, 145 — in der Hand des Büssers und neben dem, dem Ohre entströmenden Blute 1 94, 145, 202; II 272 — in der Hand der Ixnextli II 224 — in Form dieses Zeichens das Haar des Sünders bei der Itzpapälotl gezeichnet. II 264 — des Sünders bei der Xochiquetzal. I I 285 — Haar in dieser Form, Analogon der Locken (der Lichtgottheiten). II 285 — von einem Adlerfusse dem Chalchiuhtotolin dargebracht. II 272 —beim Chalchiuhtotolin angegeben II 272 — In der Hand QuetzalconatVs, des Menschenschöpfers, bezeichnet Tlagol- teotl oder tlagoUi, den obscönen Akt. II 219 — weisses, Nasenschmuck der Tlapolteotl. 1276 — repräsentiert durch den Geier (cozcaquauhtli) als Aasvogel. I 307.
cuitlatl-artige Gebilde, am Wasserstrome der Hieroglyphe chalchiuhatl, bei der Chalchiuhtlicue. 1 278 — in der Hieroglyphe des Tageszeichens atl?. I 8—9, 229 — am Ende der Scheerenarme und des Schwanzes des Skorpions. 19, 123, 229 — Jdeenverbindung mit tletl. 135, 70, 123; II 244 — mit Feuerzungen. I 61, 304, 352; I I 173 — am Wasserstrome, bei der Darstellung der Metapher atl- tlachinolli. I 229; II 238.
cuitlatl-artiges Gebilde mit Rauchwolken vor dem Munde des Sonnengottes, bezeichnet Feuer. I I 244 — am Munde der Itzpapalotl der achten Nachtstunde bezeichnet diese als ÜaelquanL II 196 — am Ende des wellig begrenzten Streifens bei der Mictecaciuatl. 1 278. ctf/>/a//-Strelfen unter dem linken Beine Xipe’s gezeichnet im Codex Fejerväry. I 271.

It seems curious to see that symbol near Patecatl:

Both the general Nahuatl word for intestine, cuitlaxculli (cuitlaxcolli), and the separate Nahuatl names for the large and small intestines – cuitlaxcoltomactli, cuitlaxculli – derived from the word cuitlatl, or excrement, which itself was subdivided by the Aztec into at least seven types (Molina 1970: 115; Sahagun 1953-1982, 10: 138). […]

If the convoluted form of the intestines linked them to negative concepts, so too did the nature and odor of their contents. As the actual embodiment of physical impurities, excrement was a logical symbol of wrong and wrongdoing. In the Mexican codices excrement often is used to indicate carnal misbehavior (Pettazzoni 1931: 192). On Codex Borgia 10 and 12, for example, it appears in the hand and/or mouth of the "sinner" (fig. 2). One of the names of the Aztec goddess of lust and sexual perversity, Tlazolteotl, was Tlaelcuani, "Eater of Ordure," while Tlazolteotl itself is usually translated as "Goddess of Filth."

Thelma Sullivan (1982: 15) argues that these names reflected the belief that by ingesting the crimes of those who confessed to her, the goddess in essence "unraveled [i.e., straightened], so to speak, the fabric of evil the penitent had woven." The sins were then transformed by her digestive system into fertilizing humus, called tlazollalli, "earth filth." On Codex Telleriano-Remensis 20, the goddess as Ixcuina, or Lady Cotton, holds in her right hand a vase with human body parts and the Aztec sign for cuitlatl, or excrement (fig. 3), and on Codex Borgia 72 and 74, spotted excretions emanate from her groin. Sahagun (1953-82, 1: 25) reports that the Aztec sinner, who always removed his clothes prior to confession, believed that Tlaelcuani "perceivist my evil odor, my vices."

The Aztec also used excrement as a literary metaphor for carnal vices in general, whether they involved sexual or some other form of excess or abuse. Aztec priests who lived in chastity, for example, are described as living a life "undefiled, without ordure, without dust, without filth" (ibid., 6: 114). The nobleman's daughter was urged not to covet carnal things with these words: "May thou not wish for experience, as it is said, in the excrement, the refuse" (ibid.: 92). Similarly, drunkenness is likened to wallowing in excrement. The drunkard born on the day 2 Tochtli (2 Rabbit) "pitched forward upon his face; he fell head first. He went wallowing in ordure, with blanched or reddened face, debauched, with hair tangled, uncombed, twisted, and matted" (ibid., 4: 11; see also p. 74). The penitent who confessed his sins was told, "perhaps thou hast retracted or thou hast suppressed, thou hast swallowed thy stench, thy rottenness, thy blackness, thy filth; ugly, stinking, rotten, it is diffused, it is known, it goeth into the land of the dead, into the heavens. Thy stench, thy rottenness are reaching the entire world" (ibid., 6: 31). And later on

But just of thy own volition thou defilest thyself, dishonorest thyself, dirtiest thyself; thou livest, thou easiest thyself into excrement, into filth. … That in which thou dost roll, in which thou dost play – the bad, the evil, the filth – hath dishonored one, dirtied one. Of thy own volition thou hast wallowed in filth, in refuse. Even as if thou wert a baby, a child, who playeth with the dung, the excrement, so hath thou bathed thyself, rolled thyself [in filth]

_{–– Cecelia F. Klein: "Snares and Entrails: Mesoamerican Symbols of Sin and Punishment", RES: Anthropology and Aesthetics, No. 19/20 (1990/1991), pp. 81-103. (jstor)}

An explanation for combining the meanings for excrement and flame may be found here:

Flames, or by extension smoke signs, are readily identificable. Seler (1902-1915, 5:504) noted their similarity with the Late Post-classic cuitlatl sign for excrement, filth, sin, and earlier Preuss (1903:216- 217) pointed out that both signs, although identical in form, do have different meanings. Two kinds of flame signs appear in the codices of the Borgia group.
One type resembles cuitlatl and the Teotihuacan flame sign (cf. flames on atltlachinolli symbol. Borgia 69), the other flame sign has two asymmetrical volutes (Borgia 69 below the foot of Xiuhtecutli). The difference in meaning is evident when both types are combined, as in the emblem of Xiuhtecutli (Cod. Cospi8) which consists of a flaming cuitlatl (Fig. 19,c).

_{–– Hasso von Winning: "The Old Fire God and his Symbolism at Teotihuacan*", Indiana 4, 1977.}

What exactly is depicted with cuitlatl seems to be of a certain variability. In the Tonalamatl plates we have two more examples: