flail: [OE] Flail is a distant relative of flagellation [15]. Both go back ultimately to Latin flagrum ‘whip’. This had a diminutive form flagellum, which in prehistoric times was borrowed into West Germanic as *flagil-. It is assumed that Old English inherited it as *flegil (although this is not actually recorded), which, reinforced in Middle English times by the related Old French flaiel, produced modern English flail. Flagellation comes from the derived Latin verb flagellāre ‘whip’. => flagellation
flail (n.)
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).
flail (v.)
mid-15c., "to whip, scourge," from flail (n.). Sense of "to move like a flail" is from 1873. Related: Flailed; flailing.