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1.
A ridiculously strong and ever-lasting oath. If broken, results in flipping off the promise-breaking party by the promisee as well as loss of respect and other unspeakable emotional, physiological, and physical consequences. More binding than a pinky promise. Abbreviated as MFP.
I flipped her off after school today because she hooked up with him, even after she Middle Finger Promised me she wouldn't.
by thecockblockersss June 14, 2010
 
2.
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."
Bro 1: Is this shit legit?!

Bro 2: I middle-finger promise!

A bond then can never be broken.
by JaRoy October 19, 2011