Green’s Dictionary of Slang

(US) a sexual sadist (poss. with an obsession with excrement/defecation), usu. as encountered by prostitutes.

1958		Murtagh & Harris Cast the First Stone 155: That’s what one dumper told me [...] ‘Honey,’ he said, ’all i want to do is to beat you up a little bit.’ [Ibid.] 304: Dumper A man who gets sexual excitement from beating a girl.
1961		T.I. Rubin In the Life 24: They’re hard rocks [...] Leave it to those other babes who don’t care about taking on the dumpers and the nuts. Dumper, you know: kicks out of beating up on you.
1972		B. Rodgers Queens’ Vernacular.
1979		Maledicta III:2 219: ‘Specialties’ have their widespread and limited jargons (dumper = sadist is rare in S & M = sadomasochistic circles, where the initials can mean anything from Sadie and Maisie to Slave and Master).