Green’s Dictionary of Slang

a general term of abuse.

1967		(con. 1950s) McAleer & Dickson Unit Pride (1981) 36: I thought he was givin’ me a break and, like a shitbum, I joined the Army.
1977		E. Bunker Animal Factory 51: He’s ten times smarter and classier than the shitbums around here.
1982		G.V. Higgins Patriot Game (1985) 200: On account of you being the bartender, shitbum.
2012		M. McBride Frank Sinatra in a Blender [ebook] So he reapeats that to whatever shit bum wrote the note, but then this guy [...] is too stupid and he fucks it up.