When a person has a sweaty arse and slides one finger along their crack collecting as much moisture as possible, then procedes to creep up behind their victim and run their finger under the victims nose depositing the sweat. Hence a bum skiff.
A: 'Fuck me dude i just gave Garry the best bum skiff ever! It must have smelt like fuckin shit!'
B: 'The poor cunt i bet he was well pissed off.'