thresh / [unvoicedth]resh/ • v. [tr.] 1. separate grain from (a plant), typically with a flail or by the action of a revolving mechanism: machinery that can reap and thresh corn in the same process | [as n.] (threshing) farm workers started the afternoon's threshing.2. variant spelling of thrash (in the sense of violent movement).

thresh separate the grain from (corn) by beating, etc. ME. threshen (XII), continuing metathetic alt. of OE. str. vb. þersċan = MLG., MDu. derschen (LG., Du. dorschen), OHG. dreskan (G. dreschen), ON. þreskja, Goth. þriskan, f. Gmc. *þersk- :- IE. *tersk-, repr. in Balto-Sl. by words denoting ‘crackle’, ‘crash’, ‘rattle’.
Hence thresher (-ER1) person or machine that threshes XIV; shark so named from the upper division of its tail with which it lashes an enemy XVII.