Green’s Dictionary of Slang

(US) the buttocks.

1930		G.A. Phelps Mental Prosperity 187: [T]o keep [‘these grown-up babies’] in good humor we must pat them on the back, even when we feel like kicking them on the bombosity [Simes:DLSS].
1932		J.L. Kuethe ‘Johns Hopkins Jargon’ in AS VII:5 329: bombosity—the buttocks.
1942		Berrey & Van der Bark Amer. Thes. Sl.
1950		WELS n.p.: (Joking words for the part of the body that you sit on) Bumbosity [...] Bombosity [DARE].
1953		M. Martin O Rugged Land of Gold 47: I had planned to sit on my bombosity and slide down the steep places [Simes:DLSS].
1969		H. Travers Voyage 69 60: ‘Hello, Passengers,’ he mouthed as though he was really saying: ‘Hello, Subjects,’ as he seated his royal bombosity [Simes:DLSS].
1989		G. Lish Mourner at Door 26: ‘Put your royal bombosity down on the royal throne and don’t you dare let me see you get up off of it’ [Simes:DLSS].
2000		Sex-Lexis 🌐 bombing the german helmet; bombosities, bombosity.
2004		(ref. to 1930s) www.wordwizard.com 🌐 The very eyedocity, you shock my modesty, / If I didn’t have my kid gloves on I’d spank your bombosity!