Green’s Dictionary of Slang

1. (US) the female breasts.

1940		J.H. O’Hara Pal Joey 102: A pair of maracas that will haunt me in my dreams.
1961		T.I. Rubin In the Life 62: Knockers, maracas, boobies. Let’s face it, Doc: I got a good shape.
1978		(con. WWII) T. Sanchez Hollywoodland (1981) 176: My poor little muchacha has a pair of maracas no bigger than pigeon’s eggs.
1989		J. Morton Lowspeak.
1990		R.G. Barratt ‘Take Derryn Hinch...Please’ in What Do You Reckon (1997) [ebook] He’s a tit man. [...] get some photos of his ex-fiancée Lynda Stoner. You wouldn’t find a bigger pair of maracas anywhere in Australia.

2. the testicles [appearance and rhy. sl.; = knackers n.].

1998

R. Puxley Fresh Rabbit.