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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.
A thousand, esp a thousand dollars; grand: A hundred and fifty thou is business (1867+)