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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.
To move wildly or violently, without accomplishing anything useful. Paging or swapping systems that are overloaded waste most of their time moving data into and out of core (rather than performing useful computation) and are therefore said to thrash. Thrashing can also occur in a cache due to cache conflict or in a multiprocessor (see ping-pong).
Someone who keeps changing his mind (especially about what to work on next) is said to be thrashing. A person frantically trying to execute too many tasks at once (and not spending enough time on any single task) may also be described as thrashing.