This is a term used often in a poker room to describe a player that is playing loose and aggressive. Having a player on tilt in a poker game can cause severe fluctuation in one's bankroll and variance in one's expected win rate.
An example of usage that I recently overheard in the Ameristar Casino's poker room, in Kansas City: "Ever since Eugene lost with his pocket rockets he has been on tilt. He is crazy. I just saw him play 3-9 offsuit"