Genesis 38:3-5

"And she conceived and bare a son; and he called his name Er. And she conceived again, and bare a son; and she called his name Onan. And she yet again bare a son, and called his name Shelah: and he was at Chezib, when she bare him."

It is of interest that Judah named Er, but that his wife named the other sons. Morris gave the names this meaning: "Er means watcher; Onan means strong; and the meaning of Shelah is not known."[11] None of these first three sons of Judah was destined to receive the birthright, in all probability, because of the pagan persuasion of their mother. There might have been a strong aversion on the part of the mother to Judah's choice of Tamar, evidently a believer in God, as the bride for her sons. Certainly, there was some reason why neither Er nor Onan consented to have a child by Tamar.