To beat or strike something or someone repeatedly and violently, or to move about wildly.
From Old English 'threscan' or 'threscian', originally meaning to thresh grain by beating it. The sense expanded to mean hitting anything violently.
The original meaning of 'thrash'—separating grain from chaff by beating—comes from ancient agriculture, and this original farming meaning is why we still 'thresh' grain today using machinery that mimics the beating action.
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