Etymology
1. From Middle English flayle, from earlier fleil, fleyl, fleȝȝl, from Old English fligel, *flegel (“flail”), from Proto-West Germanic *flagil, of uncertain origin. Cognate with Scots flail (“a thresher's flail”), West Frisian fleil, flaaiel (“flail”), Dutch vlegel (“flail”), German Flegel (“flail”). Possibly a native Germanic word from Proto-Germanic *flagilaz (“whip”), from Proto-Germanic *flag-, *flah- (“to whip, beat”), from Proto-Indo-European *pleh₂k- (“to beat, hit, strike; weep”); compare Old Norse flaga (“sudden attack, bout”), Lithuanian plàkti (“to whip, lash, flog”), Ancient Greek πληγνύναι (plēgnúnai, “strike, hit, encounter”), Latin plangō (“lament”, i.e. “beat one's breast”) + Proto-Germanic *-ilaz (instrumental suffix). If so, related also to English flag, flack, flacker.
2. Alternatively, Proto-West Germanic *flagil may be an early borrowing of Latin flagellum (“winnowing tool, thresher”), diminutive of flagrum (“scourge, whip”), from Proto-Indo-European *bʰlag-, *bʰlaǵ- (“to beat”); compare Old Norse blekkja (“to beat, mistreat”). Compare also Old French flael (“flail”), Walloon flayea (“flail”) (locally pronounced "flai"), Italian flagello (“scourge, whip, plague”). See: Latin '
plangō'.