Green’s Dictionary of Slang

(US) the female breasts.

1962		‘Greg Randolph’ Sex Goddess n.p.: The woman was really built. Most dames with bongos as large as her were pretty soft up there, he’d found.
1966		‘Rick Rand’ Sin Moved In n.p.: ‘With a pair of bongoes like she’s got [...] Man, she’s made for it’.
1986		Playboy Apr. n.p.: Breasts [...] bongos [HDAS].
1991		H. Gewirtz ‘Homer defined’ on Simpsons [Fox-TV] lisa: ‘I couldn’t afford to get you a new drum...’ [opens the card, revealing a picture of a well-endowed woman] [reading with lost enthusiasm] ‘So how about a nice pair of bongos?’ Ewww...
2000		lactam posting 11 Aug. 🌐 Ok, she’s [i.e. Britney Spears] got big bongos, but will she last? Let’s compare her against other big-breasted celebrities and try and guess if she’ll last longer than they did, or not.