a man's sexual emission, spoken of as a deposit placed into another body
This word turns up only four times, and every time it is attached to a person ("your ...", "his ...") inside the same legal sentence: a man gives it, or is forbidden to give it, into another body. It is not the ordinary Hebrew way of speaking about sex. The plain verb for that is "to lie down" (שָׁכַב), and the laws in Leviticus and Numbers reach past it for this rarer, more clinical noun whenever the point is to fix responsibility on the man and on where his emission went.
Everything turns on direction. The four laws differ only in the destination:
- into the wife of a fellow Israelite, which is adultery
- into an animal, in the two laws forbidding sex with animals
- into a woman by a man who is not her husband, in the ordeal for a wife suspected of unfaithfulness
Naming the deposit rather than the encounter lets these laws describe sexual wrongdoing the way the same writers describe everything else: as a boundary crossed and a defilement left behind, on the person and on the land. That is why the word carries far more weight than four appearances would suggest.
A sister noun from the same root, שְׁכָבָה, covers close to the same ground in the phrase usually translated "emission of semen," and Numbers 5 reaches for both words within a few verses of each other.