Underdog

1. Not statistically the favorite to win a hand. It's abbreviated as simply "dog."

Example

Very basically, if you get it all-in preflop with 99 vs. your opponent who holds AA, you are a big statistical underdog. The bullets are roughly an 80% favorite, which means your nines are a 20% underdog, and will win only one every five times.

Other Uses

The term underdog can also be used in a non-statistical manner, for example when you are heads-up with a 5-1 chip deficit or up against a top ranked player, you'd be considered an underdog to win the match.