Why is the ninth month called September?
Old English þeawes "customs, manners, personal qualities," plural of þeaw "habit, custom," from Proto-Germanic *thawaz (cf. Old Saxon thau "usage, custom, habit," Old High German thau "discipline"); 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).