Origin and history of thwart

late 14c., thwert, "from side to side, across, transversely; crosswise, across the grain," earlier in the same sense thwertover (c. 1200), overthwert (c. 1300), from a Scandinavian source, probably Old Norse þvert "across," originally neuter of thverr (adj.) "transverse, across," from Proto-Germanic *thwerh- "twisted, oblique," which according to Watkins is from PIE root *terkw- "to twist."

It is thus cognate with Old English þweorh "transverse, perverse, angry, cross," and the Proto-Germanic word also is the source of Middle Dutch dwers, Dutch dwars "cross-grained, contrary," Old High German twerh, German quer, Gothic þwairhs "angry."

The spelling shifted to -a- from 15c. From mid-13c. as an adjective, "contrary, stubborn, obstinate;" earlier overthwert, thwertover "blatant, outright" (c. 1200). As a preposition from early 15c., "across, athwart, from one side to the other."

mid-13c., thwerten, "oppose, hinder, run counter to," from thwart (adv.). Later in a physical sense of "lie across the course of, extend across, lie across so as to interrupt" (mid-14c.), also of the eyes, thwarting, "glancing sideways, looking askance." From early 15c. as "pass over or across." Spelling change from 15c. Related: Thwarted; thwarting.