What is musavada in the five precepts?

So far as I know it's defined -- defined by example -- in AN 10.176

(Impurity in speech)

And how, Cunda, does one become impure in vācā in four ways? Here, Cunda, one is musāvādī. When he has been brought to the meeting hall, in front of an assembly, among his relatives, his guild, in the midst of the royal court, on being asked as a witness: 'come, my good man, what you know, tell us', if he doesn't know, he says: 'I know', if he knows, he says: 'I don't know', if he has not seen, he says: 'I have seen', if he has seen, he says: 'I have not seen'. Thus, for his own sake, or for the sake of someone else, or for the sake of some trifle gain, he is one who deliberately{2} speaks falsely.

One is pisuṇavāco, he is one who tells there what he has heard here, as a consequence of which [there is] disunion, who tells here what he has heard there, as a consequence of which [there is] disunion. Thus dividing those who are in unity and further dividing those who are disunited,{3} delighting in dissension, devoted to dissension, gladdening in dissension, he is one who speaks a vācā that creates dissension.

One is pharusavāco, he is one who speaks such a vācā which is rough, hard, bitter to others, abusive to others, connected with anger, and conducive to non-samādhi.

One is samphappalāpī, he is one who speaks at the wrong time, who speaks of what is not actual,{4} who speaks unprofitable words, who speaks of what is contrary to the Dhamma, of what is contrary to the Vinaya; he is one who speaks words which are not worth treasuring, at the wrong time, that which is nt reasonable, not moderate, or words which are related to what is not profitable.

These, Cunda, are the four ways in which one becomes impure in vācā.

So "Musa" is one of four types of wrong speech -- false speech is wrong speech, plus there are other types of wrong speech (e.g. divisive gossip) that's wrong even though it isn't false.

The footnote for "deliberately" there says,

2. deliberately: sampajāna - here the word has a wordly meaning. Its other meaning, spiritual, is of fundamental importance in the Buddha's teaching.

The presence of the word sampajāna, in that context, might imply that speech isn't false unless it's deliberate -- or that false speech isn't impure unless it's deliberate -- I'm not sure which. Taking it literally it's even more specific:

Thus, for his own sake, or for the sake of someone else, or for the sake of some trifle gain, he is one who deliberately{2} speaks falsely.

... i.e. it's done for some kind of "gain" and maybe even in a court. So maybe more specifically equivalent to "bearing false witness against a neighbour".

I'm still not certain which is implied by DN 27:

• Is that implying that brahmins are lying deliberately?
• Or that bad karma can be made by other forms of false speech?

I'd have guessed that the speech is divisive and rooted in conceit, but what do I know ... perhaps (as implied) it was seen as rooted in some intentional deception for profit.

There's another word mentioned later ...

4. what is not actual: a-bhūta-vādī - Thanissaro Bhikkhu translates 'what is not factual', and Bhikkhu Bodhi: 'what is not fact'.

... in the fourth general category i.e. of useless and unprofitable. Maybe that's more specifically "non-factual".

Piya Tan wrote:

The fourth precept is against falsehood, against communicating what is untrue, harsh, disharmonious and useless. This is again something natural: if there is no truth, then it is no use of my writing about all this. You would have wasted your time reading this! Truth is the very basis of wholesome human communication.

So possibly musāvādā -- false speech, one type of wrong speech -- is sometimes also used to refer to wrong speech in general, any or all of the four types.

I expect that the Pali word for wrong (by which I mean, the opposite of Right, or sammā) is probably micchā -- see micchādiṭṭhi.