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.
Calm down or I'll fish slap you!
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