Fish

Slang for a weak or inexperienced player who stronger opponents expect to profit from. The metaphor is of prey — the counterpart term for a strong player is "shark".

By tendencies rather than single hands: playing too many starting hands, calling far more than raising or folding, bet sizes unrelated to the pot, and chasing draws regardless of price. One loose call proves nothing; a pattern over a session is information.

Both describe weak players. A whale is specifically one with a large bankroll who plays at high stakes, which makes them the most sought-after opponent in a game.

Because the house takes rake from every pot, money has to enter the game from losing players. Among a table of equally strong regulars nobody's edge is large enough to beat the rake, so which table a player sits at often matters more to their results than fine strategic adjustments.