His eldest servant— It is generally believed that this was Eliezer, mentioned in ch. Genesis 15:2. And as the Hebrew word זקן zaken, rendered eldest, signifies a governor, intendant, or steward of a family, (Genesis 50:7. Numbers 11:16; Numbers 11:35.) many interpreters render it, And Abraham said to one of his servants, who was the steward or governor of his house.
Put, I pray thee, thy hand under my thigh— This is the first time we meet with this manner of taking an oath in sacred Scripture, though we find it afterwards used by Jacob and Joseph. Some judicious writers have been led to believe, that this mode of swearing contained a mystery, and referred to the oath made to Abraham concerning the Messiah, who was to spring from him. While others are of opinion, that this oath was relative to the covenant of circumcision, all the privileges of which they engaged to renounce who forfeited an oath taken in this manner. But they, who assert that this practice was common in the East, and more ancient than circumcision, conceive, that it was only a sign of submission, implying, that he who used this rite acknowledged he was in the power of the other, and would be at his mercy if he kept not his oath. Grotius refers it to the custom of wearing the sword upon the thigh, Psalms 45:3 upon which the person who swore, put his hand, according to this judicious critic, with some such form of words, as, "If I falsify my oath, put me to the sword." Servants were wont thus to acknowledge their obedience, and this custom is to this day observed among some Indians. It is as much as to say, thou art subjected to me, thou art my servant.