The guff is a term the Talmud uses to refer to the repository of all unborn souls. The Talmud is the Jewish commentary on the Torah, or the Old Testament, and especially the first five books of the Bible known as the Pentateuch. Jewish tradition states that the Talmud began as oral teachings handed down from Moses that were eventually completed sometime between the 4th and 2nd century B.C.
Literally, the word guff means "body." The Talmud essentially says, "The Messiah will not arrive until there are no more souls in the guff.” The Talmud is saying that there are a certain number of souls in heaven waiting to be born. Until they are born, they wait in a heavenly repository called "the guff," and the Messiah will not arrive until every single one of these souls has been born into the physical world.
Is the idea of the guff biblical? No, it is not. Neither the Hebrew Scriptures nor the New Testament teach that there is a storehouse of souls in heaven. The Bible does not teach that souls are waiting to be attached to bodies when people are born. The Bible is not explicitly clear on when/how human souls are created, but the concept of the guff does not agree with what the Bible does teach about the origin of the soul. It is far more biblical to hold that God creates each human soul at the moment of conception, or that the human soul is generated along with the body through the physical-spiritual union of conception.