O.E.
þrescan, þerscan "to beat, sift grain by trampling or beating," from P.Gmc.
*threskanan "to thresh," originally "to tread, to stamp noisily" (cf. M.Du.
derschen, Du.
dorschen, O.H.G.
dreskan, Ger.
dreschen, O.N.
þreskja, Goth.
þriskan), from PIE base
*tere- "to rub, turn" (see
throw). The basic notion is of treading out wheat under foot of men or oxen, later, with the advent of the flail, the word acquired its modern extended sense of "to knock, beat, strike." The original Gmc. sense is suggested by the use of the word in Romanic languages that borrowed it, e.g. It.
trescare "to prance," O.Fr.
treschier "to dance," Sp.
triscar "to stamp the feet." The
thresher shark (1609) so called for its long upper tail shape.
[thresh] 
,