In court we swear to tell the truth with a hand placed on the Bible. But in the book itself, Jacob, nearing death in Egypt, asks Joseph to swear an oath not to bury him there by “put[ting] your hand under my thigh” (Gen. 47:29). Earlier in Genesis, Jacob wrestles with God, who touches “the hollow of his [Jacob’s] thigh” (32:25). “Thigh” happens to be a biblical euphemism for male genitalia; it’s from Jacob’s “thigh” or “loins” that his numerous offspring sprang.Also:
The practice of swearing an oath while touching one’s or someone else’s testicles was common in the ancient Near East (Abraham also orders a servant to do just that in Genesis 24:2). Its linguistic memory survives in our word “testify”—testis being the Latin both for “witness” and the male generative gland.
Hat tip to Alex Tabarrok of MR