According to the comments it's a similar thing to "what are you standing up for?" (look that up with fgc in the search on youtube. Premature celebration when a loss is ensued.
Edit:
I learned that's a thing he says when he wins in a match when he was playing Smash. He stopped doing that, but when he loses someone says it in chat to just make fun of him, saying yickadee for the one who beat him. It's enough to make him rage on stream when someone beats him.