Old English þeawes "customs, habit, manners; morals, conduct, disposition, personal qualities," plural of þeaw "habit, custom," from Proto-Germanic *thawaz (source also of Old Saxon thau "usage, custom, habit," Old High German thau "discipline"). According to OED, with no certain cognates outside West Germanic and of unknown origin. Meaning "bodily powers or parts indicating strength, good physique" is attested from 1560s, from notion of "good qualities." Acquired a sense of "muscular development" when it was revived by Scott (1818).
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