Kiddie-pool player

A player of a board or card game that acts immaturely. E.G. Tries to take back moves, Rubs winning in, Berates themselves or others for losing.

Kiddie-pool player: FUCK, I CAN'T BELIEVE I MISSED THAT. WHY AM I SUCH A RETARD?!?

Player 1:Go play in the Kiddie-pool.

Player 2: Yeah, you're being a real kiddie-pool player.