Green’s Dictionary of Slang

1. a womanizer, a promiscuous man; occas. of a woman (see cite c.1900).

1653		Urquhart (trans.) Gargantua and Pantagruel (1927) I Bk I 11: Julia, the daughter of the Emperor Octavian, never prostituted herself to her belly-bumpers.
1696		Motteux (trans.) Pantagruelian Prognostications (1927) II 694: Wenchers, lechers [...] stallions and belly-bumpers.
c.1900		‘Ramrod’ Nocturnal Meeting 86: I have been an outrageous belly-bumper for years, she remarked.

2. (US) the penis.

1939

G. Legman ‘Unprintable’ Ozark Folksongs (1992) I 548: That is my Neck Knacker, my Bandoliero. Neck Knacker, Breast Heaver, Belly Bumper, Quim Cracker, Thigh Thumper, &c.