1.The undisputed privilege for someone to ask unreasonable favors of his/her friends on one's birthday. The term seemed to have originated from the right to automatic shotgun on one's birthday but is also applicable in many different scenarios that may include: doing their laundry, driving them around, being their wingman and paying for food.
While the birthday guy may end up looking like a huge douche, the privilege only lasts a day so the other friends can deal with it until their birthday comes around.
2. In rare cases, the term "Birthday Rule" has also been used to satirically justify everyday asshole-ish behavior. This is not quite like the first use of Birthday Rule because the asshole-ish behavior occurred when it wasn't the guy's/girl's birthday.
1. (two friends buying lunch)
Cashier: "That'll be $10.45"
Friend One: "Dude, I can you spot me?"
Friend Two: "Ten Bucks? are you kidding me?!"
Friend One: "C'mon, it's my birthday... BIRTHDAY RULE"
Friend Two: "Ughhh, okay then..."
2. (two friends eating lunch at table)
Friend One: (starring at Friend Two's fries) "Hey Friend Two, look over there"
Friend Two: (cluelessly) "Whaaaa?"
Friend One steals the rest of Friend Two's Fries while he wasn't looking
Friend Two: "You jerk!"
Friend One: "Dude chill, Birthday Rule"
Friend Two: "It's not even your birthday!"