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