Green’s Dictionary of Slang

mufti n.

also mufty
[SE post-1900. Orig. Ind. army slang, according to Y&B f. the Mufti, a religious leader and expounder of Islamic law; thus the word ‘was perhaps originally applied to the attire of dressing-gown, smoking-cap, and slippers, which was like the Oriental dress of the Mufti, who was familiar in Europe from his appearance in Molière’s Bourgeois Gentilhomme’; note the Fr. equivalent, en Pekin, Peking-style]

1. (orig. milit.) ordinary clothes worn by someone who usu. wears a uniform to work.

[UK]‘A. Burton’ My Cousin in the Army 233: Cousin meant to cut a dash In mufty with a huge mustache.
[Aus]Sydney Gaz. 30 Nov. 3/2: [A]s soon as they could get away, the governors aid-de-camps, all dressed in mufti (i.e. plain clothes.).
[UK]Marryat Peter Simple (1911) 11: When I first looked at you I took you for some officer in mufti.
[Aus]Bell’s Life in Sydney 12 Feb. 2/4: Two mounted policemen, as they foolishly imagined, ‘in mufti’.
[UK]Trollope Three Clerks (1869) 455: He was dogged at the distance of some thirty yards by an amiable policeman in mufti.
[Aus]Courier (Brisbane) 24 june 2/5: A very reprehensible practice has hitherto prevailed of allowing the privates to parade the town in what, in military slang, is denominated as ‘mufti’.
[UK]A Son of the Soil in Macmillan’s Mag. (London) XI 389: He had still a stolen inclination for ‘mufti’; and wore his uniform only when a solemn occasion occurred.
[UK]Five Years’ Penal Servitude 155: With his back to the fireplace, behind the Chief, stood a gentleman in mufti, who [...] was a soldier likewise.
[Aus]Bulletin (Sydney) 20 Mar. 2/2: We put off mufti, donned the habiliments of swelldom, and went to honour Romany at the Royal on Saturday night, they having a dramatic entertainment.
[UK]R. Barnett Police Sergeant C 21 52: When out in mufti he did not wear the regulation boots of convict manufacture, betraying at once the constable in the civilian’s skin.
[Scot]Aberdeen Eve. Express 21 Mar 3/2: One day, taking an early stroll in mufti, he saw three or four pirates raking about.
[Aus]Truth (Sydney) 27 May 1/4: The Governor went in mufti and the others in kerridges.
L.N. Smith Lingo of No Man’s Land 55: MUFTI Tommy's term for civilian clothes.
[UK]R. Milward Man-Eating Typewriter 170: [as opposed to fancy dress] Having nanti mufti [...] I had to take the degrading Trot of Shame [...] in my Neptunus Rex schmutta.

2. one who does not wear a uniform, i.e. a civilian.

[UK]Navy at Home II 107: Some read aloud, to the great annoyance of several poor peaceable civilians, or muftis, in the room.

3. used adj. plain-clothes, thus private.

[UK]Sportsman 26 Nov. 2/1: Notes on News [...] [H]e [i.e. Gen. Robert E. Lee] has made his last charge in vain, and has some years ago gone into ‘mufti’ life .

4. (Aus.) informal clothes.

[Aus]Bulletin (Sydney) 5 July 6/2: A lady in black, with leaves in her hair and bodice, and a girl in pale-yellow silk with bands of cream moire and a wreathlet on her toplet looked simply charming, but the circle generally was a heavy mass of mufti.
[UK]A.G. Empey Over the Top 5: I was an American, in mufti, and had a little American flag in the lapel of my coat [...] but still that pointing finger made me feel small and ill at ease.