ampersand, n. — Green’s Dictionary of Slang

ampersand n.

[the fact that in late 19C nursery alphabets the symbol was usu. printed after (‘behind’) the 26 letters + the suitably curving shape of the &]

the buttocks.

[1781		C. Macklin Man of the World Act III: A consumptive, toothless phthisicy, wealthy widdow — or a shreeveled, cadaverous, neglected piece of deformity, i’ the shape of an ezard or an appersand].
1890–1904		Farmer & Henley Sl. and Its Analogues.