" Ought once was, of all things, the past tense of owe. In Shakespeare’s Henry IV, Part 1, you can see it used in both the present and the past. Mistress Quickly tells Prince Hal that Falstaff “said this other day you ought him a thousand pound.” The prince asks Falstaff, “Sirrah! Do I owe you a thousand pound?” But when you owe, you’re under an obligation. The obligation is most readily thought to be financial or transactional, but one way the word might change is for the sense of obligation to become more general. "

― John McWhorter , Words on the Move: Why English Won't—and Can't—Sit Still (Like, Literally)