1580s, muphtie "official head of the state religion in Turkey," from Arabic mufti "judge," active participle of afta "to give," conjugated form of fata "he gave a (legal) decision" (compare fatwa). Sense of "ordinary clothes (not in uniform)" is from 1816, of unknown origin, perhaps from mufti's costume of robes and slippers in stage plays, which was felt to resemble plain clothes.