Other forms: thrashing, thrashed, thrashes
To thrash is to beat somebody hard, or to move about wildly. A stick, or a fish on a riverbank.
The beating sense is old and mostly historical now, though sport keeps it alive as a way of saying beaten heavily, since a team can be thrashed six-nil. The thrashing about sense describes uncontrolled movement, so somebody can thrash in their sleep. There is a useful phrasal verb as well, since thrashing something out means arguing it through until it is settled. The word is a cousin of thresh, which is beating grain to separate it.
Proto-Indo-European
*terh₁- — to rub, turn; to drill, pierce
Proto-Germanic
*þreskaną — to thresh; to tread
Old English
þerscan
Middle English
thrasshen
English, today
thrash
From Middle English thrasshen, thresche, threschen, threschyn, threshen, thresshen, tressyn, þreisshe, þresshe, (whence the modern English thresh), from Old English þerscan, þrexan, from Proto-Germanic þreskaną (“to thresh; to tread”), from Proto-Indo-European terh₁- (“to rub, turn; to drill, pierce”). 2 Cognate with Dutch dorsen (“to thresh; to thrash”), German and lb dreschen (“to thresh; to thrash”), Yiddish דרעשן (“to thresh”), Danish tærske (“to thresh”), Faroese treskja (“to thresh”), Icelandic þreskja (“to thresh”), Norwegian Nynorsk treska, treske, treskja, treskje (“to thrash”), Swedish tröska (“to thresh”), Gothic 𐌸𐍂𐌹𐍃𐌺𐌰𐌽 (“to thresh”), Finnish rehkiä (“to drudge; to toil”); also Welsh taradr (“auger, wimble; awl, borer; drill”), Latin terō (“to rub or triturate; to wear away or wear out, consume; to tread; to press with a mill; to pass, spend”), Greek τρίβω (“to grate; to rub”), Albanian tjerr (“to spin”), be це́рці (“to rub; to polish; to pinch; to grind”), Bulgarian три́я (“to rub; to chafe; to grind”), Czech třít (“to rub; to spawn”), Polish tarć, treć, trzeć, trzyć (“to rub; to shred; to cut; to chafe; to crush; to spawn”), Russian тере́ть (“to rub; to polish; to grind”), Serbo-Croatian тр̏ти, tȑti (“to rub; to cause friction; to scour; to break, crush”), Slovak trieť (“to rub”), Slovene treti (“to crack open; to rub”), Ukrainian те́рти (“to rub; to scrub”), Armenian թուր (“sword”), sog ܕܣܦܪܛܪ (“hand towel”), hit 𒋻𒌑𒍣 (“to perform a round dance, to dance in twisting manner”), xto and txb tärk- (“to allow, let; to let go”), Sanskrit तर्कु (“spindle”).
Etymology from Wiktionary, CC BY-SA