A son of Judah, who was one of the brothers of Joseph who sold him into slavery. The story is part of a litany of disasters the brothers and their offspring visit upon themselves because...well, they're basically assholes. NOTE: masturbation is not the issue here. Onan is cheating his family by outwardly agreeing to do his duty yet privately weaseling out of it. Literally.
Tamar eventually gets her revenge and finally gets her heir in a hilarious episode that immediately follows this.

Genesis 38:1-10 (KJV)

1 And it came to pass at that time, that Judah went down from his brethren, and turned in to a certain Adullamite, whose name was Hirah.
2 And Judah saw there a daughter of a certain Canaanite, whose name was Shuah; and he took her, and went in unto her.
3 And she conceived, and bare a son; and he called his name Er.
4 And she conceived again, and bare a son; and she called his name Onan.
5 And she yet again conceived, and bare a son; and called his name Shelah: and he was at Chezib, when she bare him.
6 And Judah took a wife for Er his firstborn, whose name was Tamar.
7 And Er, Judah's firstborn, was wicked in the sight of the LORD; and the LORD slew him.
8 And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother.
9 And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother.
10 And the thing which he did displeased the LORD: wherefore he slew him also.

Once he'd started Onanism
He went onan

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