A practise usualy undertaken by homosexuals whereby a group of men sit in a circle and wank each other off in quick succession. Usualy performed with some kind of lubricant such as a big slab of butter.
A: 'Cor blimey did you see that group of puffs having a monk's circle last night in that abandoned warehouse!'
B: 'Yea those dirty fuckers i saw they had a huge slab of butter with them as well! They were at it all night!'
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!'