Contrary to popular belief, this term has nothing to do with your anal structure, nor with it's coverage in feces. A shitty bum is in fact when Christ himself decides you are worthless and lays upon you the shittiest of luck in any everyday situation. These two words act as the only consolation one can offer to a friend in such a situation.
Man 1: "Dude...I think my dick's getting smaller."
Man 2: "Shitty bum man. Shitty bum."
Man 3: "Yeah man, that's just a shitty bum. That's all it is."