Shark is slang used in brick-and-mortar casinos as well as outside them, and it is used in games where players get to play against other players, such as poker.
While the term has a slightly different meaning when used in everyday life, in the world of casinos, a shark refers to a player who preys on other players, especially the weaker ones who are, coincidentally, referred to as fish.
The term is most often used to refer to a cunning player who poses as a fish, only to take advantage of other players’ inexperience. Sharks may even go as far as to pretend that they barely know the rules of the game and will often ask for advice from other players and act all friendly, so it’s something to be wary of if you are sitting at a poker table.