Meaning

noun a swimming kick used while treading water

verb give a thrashing to; beat hard

verb move or stir about violently

originFrom Middle English thrasshen, a dialectal variant of thresshen, threshen (whence the modern English thresh), from Old English þrescan, from Proto-Germanic *þreskaną, whence also Old High German dreskan, Old Norse þreskja.

Etymology from Wiktionary, CC BY-SA 3.0