EXTRA · Parashas Vayeishev · Passuk 38:4 · not a Torah Temimah citation on this passuk
She conceived again and bore a son, and named him Onan.
Classical source (EXTRA side-ring): Yalkut Shimoni 145.
English: “And she called his name Onan” — for he brought mourning (aninut) into the world.
Derash on the verse (Yalkut) — not a formal named middah of the 13. Name etymology anticipates the narrative outcome.
Onan’s name is associated with the mourning that followed from his sin.