" Can you hear what I hear?”—now sing to the same tune “Can’s a piece of graaammar…” As grammar, it presumably started as something else, and it did: cunnan in Old English meant “know.” Ben Jonson in The Magnetic Lady has Mistress Polish praise a deceased woman for the fact that “She could the Bible in the holy tongue.” We can’t help at first suspecting a typo—she could what? But could meant, all by itself, “knew.” There was even an old expression “to can by heart” alongside our familiar “know by heart.” Modern English is littered with remnants of that stage: other offshoots of cunnan are cunning and canny, all about having your wits about you. Plus, the past tense of cunnan was a word pronounced “coothe,” from which the couth in uncouth comes: the uncouth person is lacking in know-how, as in the kind that lends one social graces. "

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