How Well Do You Know English Slang?
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 c.1600. Meaning "to make wild movements like those of a flail or whip" is attested from 1846. Related: Thrashed; thrashing. Type of fast heavy metal music first called by this name 1982.