Someone who posseses rat - like features and acts like a rat on nights out; scurrying around, stealing alcoholic drinks or dodging bar bills.
Steven: Has anybody seen Rat Boy?
Matt: No the bastard just gunned all the vod and left...
Steven: Well who's going to pay the bar bill?
Matt: Good job I have his I Phone 4 for colateral.
Steven: Ah all is well then, shall we indulge in another bottle of Grey Goose?
Matt: Indeedyo, we shall chunder like it's the end of 1999.