Sucking out is a popular slang term when a player who has very little equity in the hand manages to beat a much stronger hand. From the perspective of the losing player, this situation is known as a “bad beat”.
Let’s make an easy-to-understand example of a suck out in poker to make this a bit clearer. For instance, let’s say you went all-in with a pair of queens. Your opponent calls your bet holding pocket aces.
When the community cards come up, you get a queen and a number of lower-valued cards. This enables you to “suck out” on your opponent, who had the advantage over you when you went all-in.