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 fuck sandwich, 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).
1980		Beach Boy 89: Yeah, there’s nothing that I like more than a fuck-sandwich,’ he moaned. ‘There is,’ the boy said. ‘You like it when you’re the ham in the middle, instead of being one of the pieces of bread’ [Simes:DLSS].
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.
1997		Off-Beat Letters (NY) Mar. 45/1: I was in the middle of a hot fuck sandwich [...] While my tongue diddled its way in and out of the blonde girl’s shaved pussy, my ass was bouncing back hard and high to meet the eager strokes of the naked guy’s thrusting prick [Simes:DLSS].
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.