Green’s Dictionary of Slang

1. (Aus.) a large protruding stomach.

1728		Polly Peachum's Jests 8: You are built directly contrary to Act of Parliament, you are but two Stories high, and your Belcony [sic] hangs over your House-of-Office.
1933		R. Tate Doughman 34: Fat old forty-year Hodgeson [...] the chap with what our Boss calls a front-balcony balloon.

2. (Aus./US) the female breasts, esp. as thrust up and forward in the brassieres of the 1940s–50s.

1967–8		Baker et al. CUSS.
1984		Partridge DSUE (8th edn) 42/2: since late 1940s.
2004		(con. 1960s-70s) T. Taylor Top Fellas 49/2: Girls wore them [i.e. T-shirts] so itsy-bitsy and tight around the balcony that the [stick-on] letters would inevitably [...] fall off.