"On tilt" is a poker term used to describe someone who is letting their luck affect the way they play. For example, if someone has lost a bunch of hands in a row he or she might start playing recklessly to try to make up for it. Also, if a player has won many hands in a row, they might start to play differently as well. This playing out-of-line so to speak is called playing "on tilt."
Player: I doubt he has a great hand. He's probably only betting a lot because he's playing on tilt.