The expression “under the gun” in the context of poker is often used to indicate a situation where the player puts their opponent in a difficult position by putting pressure on them and forcing them to make difficult decisions.

When you say that an opponent is “under the gun”, this means that:

You have bet or raised, which has forced your opponent to think, and they have to choose between a call, a raise, or discarding their hand.

You have a strong hand of cards which allows you to control the course of action and have the upper hand over your opponent.

Your opponent senses that you’re active and aggressive in this hand, and they aren’t sure how best to react to your actions.

Players often aim to keep their opponents “under the gun” to maximize their chances of winning and create a profitable position for themselves in the game. This is a tactic which can be particularly effective in some situations, especially in no-limit types of poker.