1) When describing a very over-powered ablility within a game that allows certain players an unfair advantage. This is often down to user perception though. In many cases, the ability is not over-powered as long as you know how to counter it.
2) Used insultingly when a player complains that their character is underpowered. Typically they will make suggestions about how their character could be improved. If these requests are excessive, their fellow gamers will accuse them of wanting an I win button.
Sue: OMG, no way a rogue can beat a mage. Rogues should be able to use their vanish ability as often as they want without the timer.
Sam: So you want one of your most powerful abilities to be available non-stop instead of needing 5 minutes to cool-down. Basically you want an I win button?
Iwinosa is a girl who has had your back since the first day of kindergarten. A girl who has never hurt your feelings since the day you met her. A girl who is so smart, kind, and beautiful.
A verbal game played by two or more people. When one person sees a very large person in public they quietly say to their friend "I win" without pointing out the subject. The other person casually looks around untill they discover the subject and then try to find a larger person. Game goes on indefinately.
Cindo says "I win"
Cindo's friend looks around and sees very large person eating super size meal and realizes the stakes are high. Then she spots a morbidly obese lady being pushed on a Home Depot cart by two scrawny teenagers and says to Cindo, "no, I win" When Cindo finally spots the subject she says "OMG Murial, that's too much, I hate you!"