John Lightfoot on Matthew 14:7

[He promised her with an oath, &c.] This kind of oath is called by the Talmudists a rash oath: concerning which see Maimonides, and the Talmudic tract under that title. If the form of the oath were "by his head," which was very usual, the request of the maid very fitly, though very unjustly, answered to the promise of the king; as if she should say, 'You swore by your head that you would give me whatsoever I shall ask; give me, then, the head of John Baptist.'