a rule stating that you may date/hook up with one person a year who is a 4 or lower on the attractiveness scale and noone can make fun of you to your face, if and only if you claim that person as your mulligan
repeat offenses are allowed with the same person with only slight taunting after
"Dude, you dated that girl? She looks like a pig!"
A Mulligan, in a game, happens when a player gets a second chance to perform a certain move or action; usually due to lack of skill or bitter luck. A "Do-Over". Like getting an "Extra Play" in pinball due to sinking the ball before obtaining an arbitrary amount of points.
If your opening hand in Magic: The Gathering sucks bad; just call "Mulligan", reshuffle, and draw new cards.
When a dating relationship is brutally derailed by submerged emotional issues unknown to either person. If the derailer is allowed a second chance by the derailed, its a Mulligan. Even more so it accompanied by a decent bitch slap.
Wes: Dude, I can't believe all this time I didn't realize I was a sad bastard with ungodly sorrow, projecting my emotional issues on you! Please bitch slap my issues back into my subconscious and give me a Mulligan.
Laura: Fine. Take me to a nice restaurant on Friday night and it'll be fine...SMACK!