Genesis 21:28 - Ellicott's Commentary On The Whole Bible

Seven ewe lambs. — The word in Hebrew for swearing is a passive verb, literally signifying “to be sevened,” that is, done or confirmed by seven. In this ancient narrative we see a covenant actually thus made binding. Seven ewe lambs are picked out and placed by themselves, and by accepting these Abimelech bound himself to acknowledge and respect Abraham’s title to the well. Apparently this manner of ratifying an oath was unknown to the Philistines, as Abimelech asks, “What mean these seven ewe lambs?” but it is equally possible that this question was dictated by the rules of Oriental courtesy. When Abraham had picked out the lambs, it became Abimelech’s duty to ask what was the purpose of the act, which was then explained, and as soon as the lambs were accepted, the ratification was complete,