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1.
Used as an oath to convince someone that you will do something, not do something, say something, etc.

It's like a Pinky Swear except stronger.

Also called MF Swear.
"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"
by Mastersam0 June 11, 2008
 
2.
Usually stronger than a pinky swear but when done in combination with a pinky swear it cancels out the swear/promise entirely.
Making a pinky promise and a middle finger promise means Middle Finger Swear
by DictionaryMan72 September 24, 2013