Someone that you know just took a giant, fat dump, but didn't wash their hands afterwords. Meant to be a demeaning remark that forces the nasty bastard to sulk back into the bathroom to wash his hands out of embarrassment.
John: *fart* ... *toilet flushes* ... *walks out of bathroom*
Bob: Christ, John! Don't be a fucking Dumpelshitskin. Wash your filthy hands.