Question: "Why did oaths involve putting a hand under someone's thigh (Genesis 24:9)?"
Answer: In our culture, taking an oath usually involves raising the right hand or placing a hand over the heart or on a Bible. In ancient Hebrew culture, we find something a little different. Genesis 24:9 describes an odd practice that involved Abraham’s servant swearing to obey his master’s command to find a wife for Isaac: “So the servant put his hand under the thigh of Abraham his master and swore to him concerning this matter.” In Genesis 47:29, Jacob makes his son Joseph swear to bury him in Canaan, not Egypt. The same ritual is observed: Joseph is required to put his hand under Jacob’s thigh as he makes the promise. It seems strange to us, but placing one’s hand under someone else’s thigh had a symbolic purpose.
In both cases, the request is made by a patriarch nearing death. Also, both oaths deal with family matters. In the case of Abraham and Jacob, the family was blessed by God Himself (Genesis 15:5; 28:14).
The thigh was considered the source of posterity in the ancient world. Or, more properly, the “loins” or the testicles. The phrase “under the thigh” could be a euphemism for “on the loins.” There are two reasons why someone would take an oath in this manner: 1) Abraham had been promised a “seed” by God, and this covenantal blessing was passed on to his son and grandson. Abraham made his trusted servant swear “on the seed of Abraham” that he would find a wife for Isaac. 2) Abraham had received circumcision as the sign of the covenant (Genesis 17:10). Our custom is to swear on a Bible; the Hebrew custom was to swear on circumcision, the mark of God’s covenant. The idea of swearing on one’s loins is found in other cultures, as well. The English word testify is directly related to the word testicles.
Jewish tradition also offers a different interpretation. According to Rabbi Ibn Ezra, the phrase “under the thigh” means literally that. For someone to allow his hand to be sat on was a sign of submission to authority. If this is the symbolism, then Joseph was showing his obedience to his father by placing his hand under Jacob’s thigh.
Abraham’s servant kept his oath. He not only obeyed Abraham’s instructions, but he also prayed to Abraham’s God for help. In the end, God miraculously provided Rebekah as the choice for Isaac’s wife (Genesis 24).
In the New Testament, believers are taught not to make oaths, but rather to let their “yes” mean “yes” and “no” mean “no” (James 5:12). That is, we should consider all our words to have the weight of an oath. Others should be able to trust our words without requiring an oath.