ONAN (Heb. אוֹנָן; "power," "wealth"), second son of Judah and Shua (Gen. 38:2–4; 46:12; Num. 26:19). After the death of his elder brother Er, Onan was instructed by his father to contract a levirate marriage with his childless sister-in-law Tamar (Gen. 38:7–8). Onan refused to fulfill his fraternal duty, and whenever he had relations with Tamar he would let the semen go to waste (presumably by coitus interruptus, although the term *onanism can be actually applied to masturbation), thereby avoiding effective consummation of the marriage (38:9). Onan's offensive conduct was motivated by the fact that the son born of a levirate marriage was accounted to the dead brother (Deut. 25:5–6). His uncharitableness was displeasing to the Lord, who took his life (Gen. 38:10). The Judahite genealogy in I Chronicles 2:3 does not mention the death of Onan.
This story may possibly contain a historical nucleus reflecting the extinction of two clans of the tribe of Judah.
EM, 1 (1955), 155 (incl. bibl.); D.M. Feldman, Birth Control in Jewish Law (1968), 111–2.