c.1200, from Old Norse þvert "across," originally neuter of thverr (adj.) "transverse, across," cognate with Old English þweorh "transverse, perverse, angry, cross," from Proto-Germanic *thwerkhaz (cf. Middle Dutch dwers, Dutch dwars "cross-grained, contrary," Old High German twerh, German quer, Gothic þwairhs "angry"), altered (by influence of *thwer- "to turn") from *therkh-, from PIE *twork-/*twerk- "twist" (cf. Latin torquere "to twist," Sanskrit tarkuh "spindle," Old Church Slavonic traku "band, girdle," Old High German drahsil "turner," German drechseln "to turn on a lathe").
"oppose, hinder," mid-13c., from thwart (adv.). Related: Thwarted; thwarting.