“So the servant placed his hand under the thigh of Abraham his master, and swore to him concerning this matter.” Gen. 24:9
"The phrase “place your hand under my thigh” is awkward to speak, and is most likely referring to the male organ or testicles. Today we take oaths on a sacred object, such as a bible. Jews may swear on a t’fillin or a Torah scroll (Plaut).
"According to Rashi, the importance of the seed of Abraham and the promises it held for the future, made the organ the most significant object to place hands on.
"Conversely, for the servant, an oath taken in such a manner may mean the curse of sterility if the oath is not kept. (Plaut)
This story from the Old Testament is from about 2,000 BCE. Promises made by god to Abraham, and Abraham to Ellezer seem to continue to have some binding force on modern peoples. Seems a little stretch to me.