What is the meaning of Holy Spirit in Mark 3:28-29?

The Holy Spirit is the spirit of humility. To blaspheme the spirit of humility is to blame the spirit of humility. Therefore suggesting that to be humble is an evil course of action. When indeed to be humble is the requirement.

If you blame the spirit of humility you will never be forgiven, for it is "IN" humility that one finds forgiveness. The measure that God uses as the measurement of life and death is ones humility. Therefore guilt will remain, for the destination of the proud is the grave.

Micah 6:8 NKJV

He has shown you, O man, what is good; And what does the Lord require of you But to do justly, To love mercy, And to walk humbly with your God?

And this is humility

Proverbs 28:13 NKJV

He who covers his sins will not prosper, But whoever confesses and forsakes them will have mercy.

The Holy Spirit is the Holy Ghost, the third member of the Godhead (Father, Son, Holy Ghost)

Book of Mormon, Alma 39:6 gives us further insight on this scripture:

6 For behold, if ye deny the Holy Ghost when it once has had place in you, and ye know that ye deny it, behold, this is a sin which is unpardonable; yea, and whosoever murdereth against the light and knowledge of God, it is not easy for him to obtain forgiveness; yea, I say unto you, my son, that it is not easy for him to obtain a forgiveness.

What does it mean to deny the Holy Ghost?

Moroni 8:28

28 Pray for them, my son, that repentance may come unto them. But behold, I fear lest the Spirit hath ceased striving with them; and in this part of the land they are also seeking to put down all power and authority which cometh from God; and they are denying the Holy Ghost.

In order to answer your question it is necessary that we first understand the meaning of Blaspheme, and then see how that applies to the Holy Spirit, and ultimately who or what the Holy Spirit is in accordance with the Bible.

According to Merriam Webster's dictionary the verb blaspheme has two connotations. It can be either transitive are intransitive.

Transitive:

BLASPHE'ME, v.t.  is [Gr. The first syllable is the same as in blame, blasme,
denoting injury; L. loedo,   loesus; The last syllable is the Greek.,to speak.]

1. To speak of the Supreme Being in terms of impious irreverence; to revile 
or speak reproachfully of God, or the Holy Spirit. 1 Kings 21. Mark 3.

2. To speak evil of; to utter abuse or calumny against; to speak reproachfully of.

Intransitive:

BLASPHE'ME, v.i. To utter blasphemy.

He that shall blaspheme against the Holy Spirit shall not be forgiven. Mark 3.

1. To arrogate the prerogatives of God.

This man blasphemeth. Who can forgive sins but God? Mathew.9. Mark 2.

As we see above in Mark chapter 3 verse 28 the verb blaspheme is used in the intransitive form.

I have included the definition of the word arrogate since it is not a common word.

AR'ROGATE, v.t. [L. arrogo, of ad and rogo.]

To assume, demand or challenge more than is proper; to make undue claims,
from vanity or false pretensions to right or merit; as, the Pope arrogated
dominion over kings.

To this point. What we have done is to define these two verbs. Blaspheme and arrogate, now we must determine what Jesus meant in his statement. To do this we need to look at the setting, and to whom he was talking at the time.

Mark 3:22 through 26 KJV And the scribes which came down from Jerusalem said, He hath Beelzebub, and by the prince of the devils casteth he out devils. And he called them unto him, and said unto them in parables, How can Satan cast out Satan? And if a kingdom be divided against itself, that kingdom cannot stand. And if a house be divided against itself, that house cannot stand. And if Satan rise up against himself, and be divided, he cannot stand, but hath an end.

Here we find that Jesus is accused of being inhabited by Satan and it is the power of Satan within him that he uses to cast out Demons.

Jesus answers them by saying that they are not making sense, why would Satan want to cast out other Devils, since that would be exactly opposite of his goals.

Then we come to the part in which is your primary interest:

Mark 3:27 through 29 KJV No man can enter into a strong man's house, and spoil his goods, except he will first bind the strong man; and then he will spoil his house. Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation:

We need to break this down to see just what Jesus meant:

1. The strong man referes to God, and the strong man's house refers to Heaven.

2. In order to be able to have the power to be supreme it would be necessary to defeat God and then he would be able to dictate how God's power would be used.

3. In saying that the sins of man including other blasphemies (not including blasphemies against God) shall be forgiven, however, Blasphemies against the Holy Ghost will never be forgiven, and whoever does is in danger of eternal punishment.

Finally; who or what is the Holy Ghost (Spirit)?

So let's see if we can determine Biblically who this Holy Ghost is:

Matthew 1:18 Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.

Matthew 3:11 I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:

MarK 12:36 For David himself said by the Holy Ghost, The LORD said to my Lord, Sit thou on my right hand, till I make thine enemies thy footstool.

Mar 13:11 But when they shall lead you, and deliver you up, take no thought beforehand what ye shall speak, neither do ye premeditate: but whatsoever shall be given you in that hour, that speak ye: for it is not ye that speak, but the Holy Ghost.

Luke 1:15 For he shall be great in the sight of the Lord, and shall drink neither wine nor strong drink; and he shall be filled with the Holy Ghost, even from his mother's womb.

Luke 1:35 And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.

Luke 12:12 For the Holy Ghost shall teach you in the same hour what ye ought to say.

John 20:22 And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost:

When we closely examine these we see one common thread, and that is that the Holy Ghost is some sort of superior power, authority, and intelligence which comes from God.

Going back to our definition of arrogate, it appears that in this case Jesus is talking about the Scribes, ascribing the nature of God to Satan, and therefore at least in this case, The Holy Spirit refers to the nature of God, and to deny or ascribe that to any other than God is blaspheming the Holy Ghost.

This passage is in reference to Jesus casting out a demon. They accused Him of doing this with the power of Satan. Jesus replies that you can not interrupt Satan's work without binding him first, Satan is the strong man, which Jesus had to do in order to cast out the demon. He is saying that is it not logical. In addition to their argument lacking logic, they were applying the power of God to Satan and that is blasphemy.