Green’s Dictionary of Slang

(US) contemptible, unpleasant, worthless.

2000		G.V. Higgins At End of Day (2001) 92: He is a shitbum rat, and those guys never change.
2014		D.B. Flowers Bangs 19: Cops like Flowers weren’t just shit-bum headhunters out to make a collar or cause people trouble simply because they could.