The word recalls a method of taking an important vow in which one person would put his hand “under the thigh” of another (see Genesis 24:2–3, involving Abraham and his servant, and Genesis 47:29, concerning Jacob and Joseph). Nahum Sarna explains that “interpreters are unanimous that the thigh refers to the genital organ” (Genesis, JPS Torah Commentary [Philadelphia: Jewish Publication Society, 1989], p. 162). Also see R. David Freedman, “‘Put Your Hand Under My Thigh’—The Patriarchal Oath,” BAR 02:02.