Green’s Dictionary of Slang

1. an outlaw biker.

1985

E. Leonard Glitz 254: He cruised Boystown till he ran into a couple of heavyweight bikers. [...] The scummers were dressed sleeveless now that it was fifty degrees out, showing their muscle, their tattoos, their chapter insignia, Hagar the Horrible from the Funnies.

2. attrib. use of sense 1.

2000

Hip-Hop Connection Jan./Feb. 97: What happened? I had the living shit kicked out of me, scummer swine.

3. a despised person.

1984

W.J. Caunitz One Police Plaza 320: Look, scummer!