”his servant.” Despite the elevated status of this servant, he is not named. The importance of continuing the line of descendants from Abraham is thus magnified.
“put your hand under my thigh.” This is a euphemism for “take hold of my genitals.” The word “thigh” was used in the biblical culture as a euphemism for the genital organs (cp. Gen. 24:2, 9; 46:26; 47:29; Exod. 1:5; Num. 5:21, 22, 27). The taking of solemn oaths in the ancient world took many forms. Perhaps the most common one we are aware of was raising a hand (Gen. 14:22; Deut. 32:40; Ezek. 20:5, 6, 15, 23, 28, 42; 36:7; 44:12; 47:14; Rev. 10:5). Also, another common form of oath was to hold a sacred object that was somehow related to the oath or to the god of the person who was making the oath. “Gestures accompanying oath-taking are universal in the ancient world. Most frequently, they involve the raising of a hand, as in Genesis 14:22, and/or the holding of a ritual object. In later times, a Torah scroll, phylacteries, or a Bible might be held for such a purpose” (Nahum Sarna, The JPS Torah Commentary: Genesis).
Although a few scholars have asserted that Abraham asked his servant to take hold of his genitals as part of a curse that would be brought upon the servant if he did not follow through with his words, there is no evidence of that. Instead, parallels in ancient culture show us that it was a part of an oath that involved the descendants of Abraham, so grasping the genitals was deemed appropriate due to the seriousness and magnitude of the situation: after all, Isaac’s wife would be continuing the line to the Messiah.
In Genesis 47:29, Jacob requests that Joseph to hold his genitals and swear an oath to take Jacob’s body to the Promised Land and bury it. Although that certainly seems to have less to do with Jacob’s immediate descendants than in the case of Abraham’s oath, given the fact that God also promised Jacob the Promised Land (Gen. 28:13; 35:12; 48:4), and Jacob believed that he would have many descendants and then be resurrected and live among them, it makes sense that Jacob would also have Joseph swear on his genitals in that situation. Those are the only two occurrences of that practice in the Bible, and the purpose for it does not occur in ancient literature, so while swearing on the genitals was certainly not unknown, we believe it was not common, either.
Our English word “testicle” is derived from the Latin testis, which ordinarily means “witness,” and does, rarely, refer to the testicles. Some people assert that the word testis, or “witness,” and its association with “testicle” comes from the practice of placing one’s hands on the genitals and swearing on the “little witnesses.” However, there is both lexical and anthropological reasons for denying that.
From a lexical basis, although we do not know why the Latin word testis means “witness,” that is the case for most words in every language: the words, or their ancient roots, came from the Tower of Babel—the vocabulary came from God to mankind and we don’t know why most words mean what they do. And how testis came to refer to both a witness and a testicle is unknown; assumptions may be thrown around, but the fact is that we do not know.
The study of ancient Roman customs is more definitive. We have literally hundreds of ancient Latin documents about or involving oaths, and none of them refer to making an oath while holding genitals; there is simply no evidence that was done in the Roman world, which is quite good evidence that the Latin word testis for witness did not come from the custom of swearing on a person’s genitals.