'What? It can't be that bad, can it? You've not been robbin' it or something? Or you been seeing one of the girls in the gift shop and you don't wanna tell me about it?" He frowned a no, but still looked sheepish. "Come on Mickey. Spit it out" Peter said. Mickey put back his shoulders, attempting a bit of bravado. "Well..I've been doing this volunteer stuff. You know, kids and that."
You've not been robbin' it or something?
In other words:
You haven't been robbing it or something?
As a representation of speech, an apostrophe is often used to replace one or more letters of a word. It's especially common to see it used to replace the g in the -ing form of verbs: robbin', goin', seein', tryin', cryin', and so on.