A promise made while your penis is inside a person to that other person. Akin to pinkey promise but it is ten times a legit.
Justin: "Oh god I want to spend the rest of my life with you!"
Megan: "Swear?"
Justin: "Penis Promise."
Megan: "Good."
Megan: "Swear?"
Justin: "Penis Promise."
Megan: "Good."
by Fallenrook1989 July 20, 2010
by big hairy nutsack 3 August 19, 2019