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Kitto's Popular Cyclopedia of Biblial Literature
O´nan (strong, stout), second son of Judah, who, being constrained by the obligations of the ancient Levirate law to espouse Tamar, his elder brother's widow, took means to frustrate the intention of this usage, which was to provide heirs for a brother who had died childless. This crime, rendered without excuse by the allowance of polygamy, and the seriousness of which can scarcely be appreciated but in respect to the usages of the times in which it was committed, was punished by premature death (, sq.)
Kitto, John, ed. Entry for 'Onan'. "Kitto's Popular Cyclopedia of Biblial Literature". https://www.studylight.org/encyclopedias/eng/kbe/o/onan.html.