TT #28 · Parashas Vayeishev · Passuk 38:09
Parashas Vayeishev · Passuk 38:9
But Onan knew that the seed would not be his; and it came to pass, when he went in to his brother’s wife, that he spilled it on the ground, so as not to give seed to his brother.
Torah Temimah points to Yevamos 34b.
Similar to the act of Er and Onan, as it is written: “And it was, when he came in to his brother’s wife, that he spilled it on the ground.”
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The extra wording “on the ground” specifies the nature of the act.
Onan deliberately wasted seed so as not to establish offspring for his brother; the Torah marks this as the sin for which he died.