Etymology
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flail (v.)
mid-15c., "to whip, scourge," from flail (n.). Sense of "to move like a flail" is from 1873. Related: Flailed; flailing.
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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 (source also of 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).
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flagellate (v.)
"to whip, scourge," 1620s, from Latin flagellatus, past participle of flagellare "to scourge, lash" (see flagellum). Related: Flagellated; flagellating. An earlier verb for this in English was flagellen (mid-15c.; see flail (v.)).
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thrash (v.)
1580s, "to separate grains from wheat, etc., by beating," dialectal variant of threshen (see thresh). Sense of "beat (someone) with (or as if with) a flail" is first recorded 1620s. Meaning "to make wild movements like those of a flail or whip" is attested from 1846. Related: Thrashed; thrashing. As a noun from 1660s, "threshing tool;" 1840s as "a beating;" 1982 as the name for a type of fast heavy metal music.
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besom (n.)

Old English besma "bundle of twigs" (used as a broom or a flail), from West Germanic *besman- (source also of Old Frisian besma "rod, birch," Old Saxon besmo, Old High German besmo "broom, besom," German Besen, Dutch bezem), which is of unknown origin, possibly from a non-IE substrate language.

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-el (1)

instrumental word-forming element, expressing "appliance, tool," from Old English -ol, -ul, -el, representing PIE *-lo- (see -ule). In modern English usually -le except after -n-. As in treadle, ladle, thimble, handle, spindle, girdle; also compare dialectal thrashle "flail, implement for thrashing," from Old English ðerscel, Middle English scrapel "instrument for scraping" (mid-14c.), etc.

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thresh (v.)

Old English þrescan, þerscan, "to beat, sift grain by trampling or beating," from Proto-Germanic *threskan "to thresh," originally "to tread, to stamp noisily" (source also of Middle Dutch derschen, Dutch dorschen, Old High German dreskan, German dreschen, Old Norse þreskja, Swedish tröska, Gothic þriskan), from PIE root *tere- (1) "to rub, turn."

The basic notion is of men or oxen treading out wheat; later, with the advent of the flail, the word acquired its modern extended sense of "to knock, beat, strike." The original Germanic sense is suggested by the use of the word in Romanic languages that borrowed it, such as Italian trescare "to prance," Old French treschier "to dance," Spanish triscar "to stamp the feet." For metathesis of -r- and vowel, see wright.

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