In chess parlance, a fish is a term for a poor chess player. Although roughly synonymous with the dismissive colloquialisms patzer or woodpusher, there are distinctions. In general a "fish" is a player easily defeated by simple stratagems: just as a fish is oblivious to the fact that bait is attached to a hook, the player appears oblivious to the purpose of the opponent's moves. In contrast, a "woodpusher" is someone consistently uninspired in his/her own play, and "patzer" can connote an amateur whose enthusiasm is unmatched by ability.
While a fish is necessarily an unrated or low-rated player, competitors at all levels can find themselves prone to episodes of "playing like a fish"--i.e., being inattentive and/or gullible.