Green’s Dictionary of Slang

1. (also sandwich boarder) a sandwich man, carrying a pair of advertising boards around the streets [‘the doleful broken-down men employed at one shilling a day to carry pairs of advertisement boards, tabard-fashion, one on the unambitious chest, the other on the broken back’ (Ware)].

1835		Dickens ‘Dancing Academy’ in Slater Dickens’ Journalism I (1994) 255: He stopped the unstamped advertisements – an animated sandwich, composed of a boy between two boards.
1843		Leeds Times 29 Apr. 8/2: Such were bills that [...] on wooden boards were bore by walking Sandwiches.
c.1865		‘New Streets Act’ in C. Hindley Curiosities of Street Lit. (1871) 88: No walking sandwich will be allowed to parade the streets.
1869		F. Whymper Travel and Adventure in Alaska 300: The advertising ‘medium’ walking with his boards a la sandwich.
1873		Sl. Dict. 277: Sandwich a human advertising medium, placed between two boards strapped, one on his breast the other on his shoulders. A ‘toad in the hole’ is the term applied to the same individual when his person is confined by a four-sided box.
1882		Nat. Police Gaz. (NY) 24 June 3/1: [A]cting as apprentice to Moody and Sankey, carrying a sandwich for them and peddling their hynin books.
1895		Sth Wales Dly News 15 Apr. 8/6: The other ‘sandwich boarder’ [...] wore an extensive black poke bonnet.

2. (also triple-decker sandwich) a sexual threesome, involving any permutation of the sexes.

[1873		Sl. Dict. 277: A gentleman with a lady on each arm is sometimes called a sandwich. The French phrase for this kind of sandwich, l’âne à deux pannières, is expressive].
1963		in T.I. Rubin Sweet Daddy 96: They want it all [...] shack ups, gang bangs, sandwiches, the works.
1972		B. Rodgers Queens’ Vernacular 128: the one in the middle of a threesome [...] meat of the sandwich. [Ibid.] 176: sandwich 1. menage a trois [...] 2. specific threeway where two men simultaneously fuck the ass of the third partner – the meat between the two slices of bread.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 142: First time I’d been in a sandwich, funnily enough. Makes the bed very warm.
1977		(con. 1949) J.G. Dunne True Confessions (1979) 227: She told him about [...] the triple-decker sandwich the lieutenant governor liked to watch.
1979		Maledicta III:2 232: (in homosexual use) Other names and words [are] Lucky Pierre (‘in the middle’ of a sandwich, a threesome possibility).
1989		D. Waters Heathers [film script] I’m telling you man, it would be so righteous to be in a Veronica Sawyer / Heather Chandler sandwich.
2001		J. Stahl Plainclothes Naked (2002) 20: Mrs. Zank [...] winked. ‘Don’t wait up, Hilda. Tonight’s sandwich night!’.
[2004		L. Redhead Peepshow [ebook] I refused to be the meat in a ‘Have you done any modelling / I want to make you feel like a woman’ sandwich].

3. (drugs) a layer of heroin between two layers of cocaine.

1989		Hartford Courant (CT) 1 Sept. A10/3: [A]ddicts make a ‘sandwich’ by inserting two rocks of crack with heroin powder between them.
2001		ONDCP Street Terms 18: Sandwich — Two layers of cocaine with a layer of heroin in the middle.