mufti (n.1)

1580s, muphtie "official head of the state religion in Turkey," from Arabic mufti "judge," active participle (with formative prefix mu-) of afta "to give," conjugated form of fata "he gave a (legal) decision" (compare fatwa).

 "ordinary clothes, citizen's dress worn by officers when off duty," a British army term from India, 1816, of uncertain origin, perhaps from mufti (n.1) in reference to mufti's costume of robe and slippers in stage plays, if this was thought to resemble plain clothes.