Abarim Publications' online Biblical Hebrew Dictionary
The feminine noun זפת (zepet), meaning pitch, is a bit of an orphan. There is no associated verb extant in the Hebrew Bible, nor are there other words known that derive from a joined root. And so it is assumed that our word is a loan word from either Aramaic or Egyptian, but that's conjecture at best.
If this noun was a proper Hebrew word, it would have been derived from a verb. Perhaps this verb was of a similar meaning as כפר kpr, meaning to cover over or even to atone.