English: R’ Yochanan said: Whoever emits seed in vain is liable to death, as it is stated: “And what he did was evil in the eyes of Hashem; and He slew him also.”
On the daf / in context: Niddah 13a derives from Onan’s death that wasting seed is a capital-level offense in the heavenly court.
Type of Learning
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The death of Onan is read as the paradigm for the severity of this act.
What Is Learned
Emitting seed in vain is so severe that the Torah records death as its consequence in Onan’s case.