Definition of fish
The fish (fish in French) is also called pigeon, donkey, monkey, etc.. The fish is the bad player at the table, the one who doesn't know the basic strategy. When a player is called fish, it's an insult. Not all players at a poker table have the same skills. Professionals who have respect for beginners or less experienced players will call them "recreational players" or "recreational players". But when you want to insult a player, you will call him a fish, and usually he doesn't know he is a fish. There is an adage in poker that "80% of poker players think they are better than 80% of poker players".
Don't tell the fish that he's fish.
If you're a pro player and you've just taken a serious badbeat by a player you call a "fish", don't tell him he's a fish, tell him "nice hand" or "well played" instead. Treating a player as a fish could have several negative effects on you. For example, you could lose your chat privileges in your online poker room, you could cause your opponent to study harder to the point where their knowledge of the game exceeds yours, you could cause your opponent to leave the table, and so on. Other than breaking down, there is no advantage to treating a player of fish.