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.