Proclaiming a victor, either yourself or other, in a competition. It is a derivative of the normal "winner" term being different only in that it is normally shouted in order to make it heard over everything and everyone else.
You singlehandedly regained control of a commercial aeroplane after the pilot passed out and you also successfully thwarted a criminal mastermind's attempt
to launch a lethal salvo of nuclear missiles (that would inevitably incite WWIII), just by landing the plane onto the mobile sattelite dish that was meant to send out the launch signal in T- 2 seconds before your uncanny arrival. In that kind of instance, the only appropriate phrase for you to shout would be "WINNNAARR!"