implement for threshing grain, c.1100, perhaps from an unrecorded Old English *flegel, which, if it existed, probably is from West Germanic *flagil (cognates: Middle Dutch and Low German vlegel, Old High German flegel, German flegel), a West Germanic borrowing of Late Latin flagellum "winnowing tool, flail," in classical Latin "a whip" (see flagellum).

mid-15c., "to whip, scourge," from flail, n.. Sense of "to move like a flail" is from 1873. Related: Flailed; flailing.