To beat or strike violently, or to move about wildly. Can also mean to defeat decisively in competition.
From Old English 'threscan' meaning 'to beat, thresh grain.' Related to 'thresh,' the agricultural process of beating grain to separate it from chaff.
The connection between 'thrash' and 'thresh' reveals how violent metaphors often come from peaceful agricultural work - the same beating motion that separates wheat from chaff became a word for any violent striking.
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