The adjective is derived from Early Middle English thwert, thwerte, thuart, thurt, thurte, thwart, thwarte, twart, twarte, twhart, twhert, twort, þuert, þwerrt, þwert, þwerte, ðwert (“crosswise, transverse; counter, opposing; contrary, obstinate”), borrowed from Old Norse þvert (“across, athwart”), originally the neuter form of þverr (“across, transverse”), from Proto-Germanic *þwerhaz (“cross; adverse”) (altered or influenced by Proto-Germanic *þweraną (“to stir; to swirl; to turn”)), from Proto-Germanic *þerh-, probably from Proto-Indo-European *terkʷ- (“to spin; to turn”).
The English adjective is cognate with Danish tvær (“sullen, sulky”), Gothic 𐌸𐍅𐌰𐌹𐍂𐍃 (þwaírs, “angry”), Middle Dutch dwers, dwars (modern Dutch dwars (“crosswise, transverse; slantwise, askew; stubbornly disobedient”)), Norwegian tvert, tvært, Old Frisian þweres, dwers (Saterland Frisian twars, West Frisian dwers, dwerz (“across, to the other side of; beyond”)), Middle Low German dwers, dwars (Low German dwars (“contrary; cross-grained”)), Old English þweorh (“transverse; perverse; angry, cross”), Old High German twer (Middle High German twer, quer, modern German quer (“crosswise; cross”)), Swedish tvär (“across, transverse; of a curve: sharp; immediate, sudden; grumpy, stubborn”). It is related to queer.
The adverb is derived from Middle English thwert, ywerte (“crosswise; across the grain”); the Middle English Dictionary suggests the adverb was derived from the adjective, while the Oxford English Dictionary notes that the adverb is attested earlier than the adjective.
The verb is derived from Middle English thwerten, thwert, thwarten, þwerten (“to lie across; to oppose, to thwart”), and further from the adverb and perhaps also the adjective.
Noun sense 1 (“a seat across a boat on which a rower may sit”) may be derived from the adverb or adjective, from the position of the seat across the length of the boat, while noun sense 3 (“(rare) an act of thwarting”) is derived from the verb. Compare Middle English thwert (“in in thwert: crosswise”), from the adjective.