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.