Definition of underdog
Whether in poker or any other competition, underdog is the negligee. There is the favorite on one side, the one with the best chance of winning the game, and on the other side, the one who will win the game in a smaller proportion of the time. The underdog can be a big underdog, i.e. its chances of winning are very low, or a small underdog, i.e. its chances of winning are a little lower than the favorite, but still interesting.
The underdog in poker
In poker, we can say that a player is underdog or a poker hand is underdog. If there is a head-to-head duel between two players, the best of the two players will be called the favorite while the other will be underdog. For poker hands, if two players are all in in a hand and one has QQ and the other AA, we will say that QQ is underdog. underdog does not mean that we never win, but that we are not the favorite to win.