Green’s Dictionary of Slang

(a) (US short order, also mystery brown) a plate of corned beef hash; also attrib.

1885		North Amer. Rev. Nov. 434: In the slang of the New York common restaurant waiters a plate of hash [is known as] ‘mystery.’.
1889		Pittsburgh Dispatch (PA) 26 Jan. 9/7: ‘Mystery brown’ is corn beef hash.
1890		Lafayette Advertiser (LA) 12 Apr. 4/2: He pitched into an inviting plate of ‘mystery’.
1899		Atlanta Constitution 17 July 5/4: ‘Plate mystery’ brought plain corned beef hash.
[1900		Sydney Sportsman (Surry Hills, NSW) 17 Oct. 1/4: Handicapping [...] is as a dark and unfathomable a mystery as is a 3d hashouse Irish stew].
1906		C. M’Govern ‘Soldier Sl.’ in Sarjint Larry an’ Frinds n.p.: mystery:— Corned beef hash.
1918		T. Thursday ‘Nearly Over’ in Top-Notch 15 Apr. 🌐 Did you taste that hash? Some mystery, take it from me!
1926		Maines & Grant Wise-crack Dict. 9/2: Hashslinger – Mystery man in a restaurant. [Ibid.] 11/1: Mystery – Hash.
1940		Waukesha (WI) Freeman 24 Jan. 3?/3: ‘Kitchen mystery’ or ‘sweep up the kitchen’ – hash.
2003		The ‘Old’ Oxford Menu at The Oxford Saloon & Cafe Established 1883 🌐 ‘Mystery’ Hash.

(b) see mystery meat

(c) of food, i.e composed of unknown ingredients, or of an unknown variety.

1913		Day Book (Chicago) 16 July 12/1: The waiter slipped him a can of red ink and a bowl of mystery soup.
1946		‘Josephine Tey’ Miss Pym Disposes (1957) 102: [T]urning over with a fastidious fork the vegetable mysteries on her plate.

(d) (US) a chocolate and vanilla sundae.

1931

O.O. McIntyre New York Day by Day 12 May [synd. col.] Chocolate and vanilla sundae — mystery.

(e) see bag of mystery under bag n.¹

(a) a young girl.

1879		London Life 24 May 6/2: Trade is slack in everything but ‘mysteries’.
1901		W.A. Sun. Times (Perth) 2 June 1/1: The opportunity of hugging the buxom mystery proved disastrous to one Kalgoorlie Johnny.
1958		F. Norman in Vogue Oct. in Norman’s London (1969) 31: He would hang about all day in the hope of picking up some young mystery.
1962		R. Cook Crust on its Uppers 50: It [...] means the little mystery’s woken up when she wasn’t supposed to.
1989		J. Morton Lowspeak 102: Mystery – a girl, specifically a virgin.

(b) an unknown young woman, often one recently arrived in London from the provinces.

1937		J. Worby Other Half 278: Mystery, a girl who is down and out, come to London to look for a job.
1956		‘Charles Raven’ Und. Nights 107: It seems that Lew had taken up with a mystery, which is the underworld expression for one of those teen-age girls who drift into London from the provinces.
1965		G. Melly Owning Up (1974) 41: The clientele was both seedy and dodgy; male and female prostitutes, layabouts and mysteries, small-time tearaways.
1970		G.F. Newman Sir, You Bastard 21: All we gotta get now is a couple of mysteries.
1977		D. Powis Signs of Crime 194: Mystery An adolescent female absent from her home, approved school or Borstal and ‘floating’ around the streets.

(c) a young prostitute.

1959		F. Norman Fings I i: Yeah, and on top of that there was a couple of mysteries on the underground selling short times like ’ot cakes.
1960		C. MacInnes Mr Love and Justice 19: Two or three [clients] a day [...] What do you take me for – mystery?
1963		F. Norman in Show Jan. in Norman’s London (1969) 141: Clients [...] wandered round the back streets of Soho peering down dark alleys in the hope of catching some baby mystery who might show her face.
2024		J. Meades Empty Wigs (t/s) 208: She was a randy little mystery. But choosey.