Fishmalk is a term used to refer to a stereotypically wacky Malkavian, the type who hits someone in the face with a fish and runs away hooting. Since Malkavians are all insane, the clan tends to attract a type of player who wants to be wacky or otherwise "chaotic neutral". The specific origin of the term has been attributed to an illustration in the Vampire: The Dark Ages Rulebook, which featured a "knight in his undies bleeding into a goblet while some guy fondled a dead fish."
Two other common creation stories involve:
- Malkavians that believe themselves to be fish
- The phrase "If you don't put down that fish, Malk..." (say it out loud if you don't get it)
An arguably loving homage to them appears in Act 6 of SOI's Gehenna: The Musical. One final reference can be found in the descriptive quote for Dominate Malkavians in Vampire: The Masquerade 20th Anniversary Edition, where the vampire states that they will not "flip out and start hitting people with fish".
In White Wolf's Dudes of Legend: How to Be Fucking Awesome there is a brief mention when the text says: "“Werewolves are fishmalks, even though that only makes a little bit of sense, and every mage can dodge bullets.” You'll have to read the book to understand why this was said.
"Fishmalk" can be seen as a sort of insult to a player's ability to roleplay, as it is quite pointlessly silly and not ideally suited for the World of Darkness.