The phrase occurs in the Bible in the following context:
Jesus had been "casting out spirits," and some of the Pharisees accused Jesus of doing so by the power of "Beelzebul, Prince of Demons."
Shortly after this Jesus says that all sin and blasphemy against the Son (Jesus) will be forgiven, but that blasphemy against the Holy Spirit shall not be forgiven.
Many feel that the comment is directly related to the accusation, and that "blasphemy against the Holy Spirit" is what the Pharisees were doing when they attributed something done by the power of the Holy Spirit to something done by the devil.
Others suggest that it amounts to acknowledging that God is sovereign and that He offers forgiveness, and rejecting that forgiveness anyway. The main argument for this seems to be that in every other case where forgiveness is mentioned it's suggested that anyone who asks forgiveness will be forgiven. In this interpretation, it's not so much that the sin itself is inherently unpardonable as that anyone who commits it will not ask forgiveness, since it's already been rejected.