Originating from "shit bastard", however the meaning is slightly changed... a shit bastard is a person who is annoying for some reason, the phrase "shitbas" is used for describing something or an explanation of annoyance or displeasure.
Tom: "Man, i lost the car keys, way out there in the jungle"
Tim: "Argh Tom, you shit bastard, how could you do that?!"
"Crap, my CD is broken!"
"Na, don't matter, it was shitbas anyway..."
"Shitbas! Just hit my faffing head on the door!"