The expession used to describe one's fingers after one has satisfied their habit of picking their anal passage. The individual associated with this habit is Sir Steven of Stechford. Rumour has it, the bum fingers of the master of this art, can be caught by scent from over 200 miles away.
"Steve, get your bum fingers away from me."
"Steven, I can smell your bum fingers"
"Where is Steven?"
"Probably sitting at home with his fingers up his bum"