John Gill Commentary Genesis 38:4

"And she conceived again, and bare a son; and she called his name Onan." — Genesis 38:4 (ASV)

And she conceived again, and bore a son As soon as she well could: and she called his name Onan; the first son Judah gave the name to, but his wife named this, so called from grief or sorrow; the reason of it, according to the above Targum, was, ``because his father would mourn for him;'' he was a Benoni, see (Genesis 35:18) , whose sin and immature death caused sorrow.