Old English þwong, þwang "narrow strip of leather" (used as a cord, band, strip, etc.), from Proto-Germanic *thwang- (cognates: Old Norse þvengr), from PIE root *twengh- "to press in on, to restrain" (cognates: Old English twengan "to pinch, squeeze"). As a kind of sandal, first attested 1965; as a kind of bikini briefs, 1990.