Commentaire sur La Genèse 38:3

AND HE CALLED HIS NAME ER. Judah called his son Er, said name being derived from the expression, Stir up (‘Or’rah’) Thy might.¹³⁰Psalms 80:3. His wife called the name of the second son Onan,¹³¹A word which suggests grief and mourning. Ramban makes the point that the name Judah chose for his son can easily be surmised, as it suggests strength. But why his wife should choose a name like “Onan” is not indicated. but Scripture does not relate the reason for this name. Now it is possible that she experienced difficult labor, for it is customary for women to name their children after such an experience, as did the mother of Jabez who so named him, saying: Because I bore him with pain.¹³²I Chronicles 4:9. The name “Yavetz” contains the Hebrew letters of atzev (pain). And so did Atarah, the mother of Onam,¹³³Ibid., 2:26. [call him by the name Onam on account of her difficult labor], the name being derived from the expression, And the people were ‘k’mithon’nim’ (as murmurers);¹³⁴Numbers 11:1. Wherefore doth a living man ‘yithonen’ (complain)?¹³⁵Lamentations 3:39. This is similar in expression to ben oni (the son of my sorrow)¹³⁶Above, 35:18. mentioned in the case of Rachel. Judah was not particular about changing Onan’s name as his father Jacob had done.¹³⁶Above, 35:18.
In Bereshith Rabbah¹³⁷85:5. our Rabbis said, by way of explaining the name Er, that he was destined to be thrown off (she’hu’ar) from the world.¹³⁸Since, as Scripture relates, he died on account of his sin. (Verse 7, and see Ramban there.) Now this is not to say that such was Judah’s intent. However, the Rabbis made their exposition since the names indicate the future.

ותהר ותלד, Yehudah named the first son, whereas his wife named the second son. This appears to have been the custom at that time. The third son actually should have been named by Yehudah again, but seeing that Yehudah was in Keziv at the time of his birth, Yehudah’s wife named him. This is why the Torah explains the departure from the norm by writing of Yehudah that he was in Keziv.