I'm reading H. Beam Piper's 1950's era story "The Edge of the Knife" where he refers to a military man wearing civilian clothes as "the major in mufti."
I looked up "mufti" and came up with "a professional jurist who interprets Muslim law ," which seemed to make no sense.
The alternative definition makes more sense: "ordinary dress as distinguished from that denoting an occupation or station mufti
>; especially: civilian clothes when worn by a person in the armed forces"
So, the major is in mufti because he is wearing his "civies" rather than his uniform.
I wonder why "mufti" has such seemingly different meanings.
The etymology dictionary is not particularly helpful:
mufti (n.) 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" (cf. 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.