What is the blasphemy of the Holy Spirit? In Matthew 12:31-32, Jesus says:


  Therefore I tell you, every sin and blasphemy will be forgiven people,
  but the blasphemy against the Spirit will not be forgiven. And whoever
  speaks a word against the Son of Man will be forgiven, but whoever
  speaks against the Holy Spirit will not be forgiven, either in this
  age or in the age to come. (Matthew 12:31-32 ESV)


In the KJV, this is called "blasphemy against the Holy Ghost".

What does this refer to?

PS: I have taken a look at a related question(Are there unforgivable sins?), but I would like more answers to this specific question.
To understand blasphemy against the Spirit, one first needs to understand what blasphemy is. The Catechism of the Catholic Church defines it thusly:


  Blasphemy is directly opposed to the second commandment. It consists in uttering against God - inwardly or outwardly - words of hatred, reproach, or defiance; in speaking ill of God; in failing in respect toward him in one's speech; in misusing God's name. St. James condemns those "who blaspheme that honorable name [of Jesus] by which you are called." The prohibition of blasphemy extends to language against Christ's Church, the saints, and sacred things. It is also blasphemous to make use of God's name to cover up criminal practices, to reduce peoples to servitude, to torture persons or put them to death. The misuse of God's name to commit a crime can provoke others to repudiate religion.


Basically, it's going against God's will and treating Him with anything less than the respect and love He deserves.

The Holy Spirit, being one part of the Holy Trinity, is what confers the gift of grace upon people. So, to blaspheme against the Spirit is to willfully reject God's grace and forgiveness. Or as the Catechism puts it:


  "Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven." There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss.


The analogy I've heard is that it's to know God exists and know His power, but to believe you can do better. If you're familiar with the story of the fall of Lucifer/Satan, it's essentially his actions in a nutshell: he knew God exists and he knew God's divine grace, yet he openly rejected it. 

And thats why it's so unforgivable: God can't forgive you or save you if you reject His ability to do so.