Green’s Dictionary of Slang

1. the stomach of a pregnant woman.

1895		Crowe Aus. Sl. Dict. 7: Bay window [...] enceinté.
1896		Farmer Vocabula Amatoria (1966) 35: Besace, f. 1. The stomach of a pregnant woman; ‘a bay-window’.

2. (also bay front, bay-window front) a man’s fat stomach; thus the fat man himself.

1879		Cimarron (NM) News 27 Nov. 3: Since his bay window began to form [W&F].
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1895		Crowe Aus. Sl. Dict. 7: Bay window, big stomached.
1900		Houma Courier (LA) 8 Sept. 8/3: An’ how fat he is; look at the bay window he pushes —the pity o’ it.
1908		B. Fisher A. Mutt in Blackbeard Compilation (1977) 90: [illus. of very fat man] By nominating Mr. Baywindow and his twin brother our tonnage would compare favourably with Mr. Daft.
1923		C. MacArthur Rope n.p.: You don’t see no bay window here [W&F].
1930		(con. 1900s–10s) Dos Passos 42nd Parallel in USA (1966) 137: It’s the damned old bay window’s fault. He chewed the rag so much about politics all day.
1932		(con. 1910s) J.T. Farrell Young Lonigan in Studs Lonigan (1936) 27: He pursed his fat lips, rubbed his fat paws together and suavely caressed his bay front.
1945		A. Kober Parm Me 81: He’s got a little bay-window front, and he’s inclined along the lines of being short.
1956		H. Whittington Forgive Me, Killer (2000) 52: He was hitching his trousers up over his bay window.
1961		H.C. Woodbridge ‘Misc.’ in AS XXXVI:3 227: bay front, n. Variant of bay window.
1991		R.G. Barrett Between the Devlin 139: ‘[Y]ou are developing quite a noticeable bay window’.
2005		E. White My Lives 89: He’s going to have a heart attack with that big bay window of his.