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 6, 2004
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
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 17, 2008
Being slapped during intercoarse by a girl; after she has rubbed herself into a frensy.
Mate, I had so much to drink that I nearly fell asleep as she was riding me; that was until she fish slapped me. I wasn't sure if it was the slap or the stench that woke me up!
by incinerator March 2, 2012
When a guy is giving a girl oral sex and shakes his face back and forth in her vagina
"She got pretty pissed when I started fish slapping her"

by Juggs April 3, 2006
To slap someone with a fish. Often used in nonsense shows, movies and stories.
Cuzzin Red- "Stawp fish slappin' those inocent people! They never dun anything ta y'all!"

The Masked FIsh SLapper- "I will go around, fish slapping whoever I want!"
When one wets their non-errect penis and wiggles and smacks it in their partners face.
Resembling a fish out of water.
Steve: Yo, last night I was doin some real freaky shit wth my bitch.
Ron: what you do?
Steve: I did the Russian Fish Slap.
Ron: Thats pretty intense man.
by jbird1 July 30, 2011