mufti

mufti \MUHF-tee\, noun:

1. civilian clothes, in contrast with military or other uniforms, or as worn by a person who usually wears a uniform.
2. a Muslim jurist expert in the religious law.
3. (in the Ottoman Empire) a deputy of the chief Muslim legal adviser to the Sultan.
4. (initial capital letter) Grand Mufti.

The men in mufti take the field in Goodbye Mexico, leaving behind the armed warriors and those who couldn't find where to getmufti.
-- Phillip Jennings, Goodbye Mexico, 2007

"Zdrastvuyte kak pozhivaete horosho spasibo" Entwistle rattled off in excellent imitation of Russian speech--and indeed he rather resembled a genial Tsarist colonel in mufti.
-- Vladimir Nabokov, Pnin, 1957

Mufti came into English in the late 1500s from the Arabic word of the same pronunciation. The word literally meant a person who delivers a judgment. The sense of "civilian clothing" arises from the legal adviser being a civil official who did not wear military garb.