Genesis 38:7-9

Expanded Bible

⁷ But Er, Judah’s ·oldest son [firstborn], did ·what the Lord said was evil [^Levil in the eyes/sight of the Lord] so the Lord killed him. ⁸ Then Judah said to Er’s brother Onan, “Go and have sexual relations with your dead brother’s wife. It is your duty to ·provide children for your brother in this way [^Lraise up a seed for your brother; Deut. 25:5–10].”

⁹ But Onan knew that the ·children [^Lseed] would not belong to him, so when he ·was supposed to have sexual relations with [^Lhe went to] Tamar he ·did not complete the sex act [^Lspilled his seed on the ground], ·making it impossible for Tamar to become pregnant and for Er to have descendants [^Lso that he did not give a descendant/seed to his brother].