The finger that is frequently used to pleasure the anus.
Oh no, i've sprained my bumfinger
John's bum finger looks brown. He must hae been using it.
The reclassification of a human finger/owner of the finger following a skin graft taken from the buttocks of the individual in question. Usually but not exclusively as the result of an accident. To be uttered to the tune of "Goldfinger".
A: "Oh no, he cut off the tip of his finger, he might have to have a skin graft"
B: "Don't they usually take the skin from your arse?"
A: "what and have a bumfinger? ...