John Gill Bible Commentary

Neither shalt thou swear by thy head,.... This also was a common form of swearing among the Jews: take a few instances. "If anyone is bound to his friend by an oath, and says to him, vow unto me , "by the life of thy head"; R. Meir says (u), he may retract it; but the wise men say, he cannot.'' Again (w), a certain Rabbi said to Elijah, "I heard "Bath Kol" (or the voice from heaven) mourning like a dove, and saying, woe to my children; for, because of their sins, I have destroyed my house, and have burnt my temple, and have carried them captive among the nations: and he (Elijah) said unto him , "by thy life, and by the life of thy head", not this time only it says so, but it says so three times every day.'' Once more (x), says R. Simeon ben Antipatras, to R. Joshua, "I have heard from the mouth of the wise men, that he that vows in the law, and transgresses, is to be beaten with forty stripes: he replies, blessed art thou of God, that thou hast so done, , "by thy life, and by the life of thy head", he that is used to do so is to be beaten.'' This form of swearing is condemned, for this reason, because thou canst not make one hair white or black: which shows, that a man's head, nor, indeed, one hair of his head, is in his own power, and therefore he ought not to swear by it; as he ought not to swear by heaven, or earth, or Jerusalem, because these were in the possession of God. Some copies read, "canst not make one white hair black". (u) Misn. Sanhedrim, c. 3. sect 2. (w) T. Bab. Beracot, fol. 3. 1. (x) Derech. Eretz, c. 6. fol. 18. 2.