Green’s Dictionary of Slang

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

1822		‘A. Burton’ My Cousin in the Army 233: Cousin meant to cut a dash In mufty with a huge mustache.
1833		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.).
1834		Marryat Peter Simple (1911) 11: When I first looked at you I took you for some officer in mufti.
1848		Bell’s Life in Sydney 12 Feb. 2/4: Two mounted policemen, as they foolishly imagined, ‘in mufti’.
1857		Trollope Three Clerks (1869) 455: He was dogged at the distance of some thirty yards by an amiable policeman in mufti.
1862		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’.
1865		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.
1877		Five Years’ Penal Servitude 155: With his back to the fireplace, behind the Chief, stood a gentleman in mufti, who [...] was a soldier likewise.
1880		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.
1888		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.
1893		Aberdeen Eve. Express 21 Mar 3/2: One day, taking an early stroll in mufti, he saw three or four pirates raking about.
1894		Truth (Sydney) 27 May 1/4: The Governor went in mufti and the others in kerridges.
1896		Regiment 4 Apr. 4/2: One morning early, the General was taking a stroll in mufti.
1918		L.N. Smith Lingo of No Man’s Land 55: MUFTI Tommy's term for civilian clothes.
1936		Wodehouse ‘The Amazing Hat Mystery’ in Young Men in Spats 103: [A] uniformed little blighter whom he recognized as Bodmin’s errand-boy and an equally foul kid in mufti.
2023		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.

1831

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.

1868

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.

1890		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.
1917		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.