To hit someone with a fish, usually to the face. In real life(IRL), this is considered fairly extreme in most groups and not terribly common. In internet chat, especially IRC, it's a very common way to show mild disapproval/discust/boredom. It was included as a (the only?) default point-and-click action in mIRC (a popular Windows IRC client) further increasing it's popularity. The default action (changeable, randomizable, etc) when fish slap is selected is currently "X slaps Y around a bit with a large trout".

When discussing the practice, many people tend to bring up Monty Python's Fish Slapping Dance, a popular sketch from Monty Pythons's Flying Circus. Though certainly not the only fish slap in the media, it's most likely one of the most famous.

A term used to reference cannabis/marijuana/weed.

To beat relentlessly with a fish, usually with a large trout or tuna

when someone needs to calm the front door down....and they wont stop so you have to slap them with a fish..perferably in the face.

A fishslap is where a lifeform slaps another lifeform with a fish; usually homo sapiens.

Fish slap is when u shove ur whole cock down ur girls Throught and and pull it out and slap her with it