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3.
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.
Lys was being overdramatic so we had to fish slap her.
by boots wit furr February 16, 2008
 
1.
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!
by Patrik Lundin October 05, 2004
 
2.
To beat relentlessly with a fish, usually with a large trout or tuna
That dude was pissing me off, so I FISHSLAPPED him with a tuna.
by weirdo June 27, 2003