In poker, the term underdog refers to a hand that is statistically less likely to win after all community cards are dealt.
For example, when AQ is up against AK, it is an underdog, as it only has a 30% chance of winning by the river.
The term underdog can also sometimes be used to describe a player who has a very bad chance of winning in a certain game because their skill level is subpar compared to that of the other players. When referring to this, think of a fish or a whale.