The Fingering of somebody's ass crack to maximize pleasure. Can results in nails being snapped off and internal bleeding. Bum Finger ussually takes place after a good bout of fart eating leap frog which creates of lots of sexual energy and vibes.
"Catch my fart in your mouth Amelie then jump over me and fart in mouth, it really makes me horny!"
"OK! But can we do Bum Finger afterwards because i've been priming my fingers all weak by sticking them in my own shit"