In modern use, a word that generally refers to masturbation, derived from Hebrew אונן Onan, a symbolic character in the Bible who "spilled his semen on the ground," an act that prompted God to kill him. Thus, the word "onanism" developed in Judaism and Christianity as a reference to forbidden sexual acts, usually masturbation.
"And ער Er, Judah's firstborn, was רע ["sexually impure, wicked"] in the sight of the LORD; and the LORD slew him. And Judah said unto אונן Onan, Go in unto thy brother's wife, and marry her, and raise up זרע seed to thy brother. And Onan knew that the זרע 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 זרע seed to his brother. And the thing which he did displeased the LORD: wherefore he slew him also." —Genesis 38
"The hand that reaches below the navel should be chopped off." —Talmud, Niddah 13a