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