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.
*in a crowded stadium*
Mr. Smith is the WINNAR!!!!!
*at the end of a video game match*
Person1: That was stupid as..
Person2: WINNAR!!!! *pointing to himself*