Similar to a pinky promise, except with he use of the middle fingers. Usually a bond between two bros. It's a promise, acknowledging, "if I break this, I'm fucked."
"dude if you slap that girl's ass I'll pay you 5 bucks>"
"ok, Middle Finger Swear on it."
***slap***
"ha ha newb now you have to pay, we MF Swore on it"
"dammit"