A person or character who behaves in a "random" or "wacky" manner in an attempt at humor, to the annoyance of those around them.
The term is believed to have originated from the "Vamprie: the dark ages" rulebook for the "the dark ages" roleplaying game, as a portmanteau of the word fish (notorious for their wacky/randomness) and Malkavian (a clan notorious for every member being at least partially insane.)
A 'Chaotic Random' character in TTRPGs, typically played by someone who doesn't really care about the game. While the other players work to gain the trust of the king, the Fishmalk runs up and slaps him with a duck. They are no fun to play with.