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1.
Sarcastically used to point out an unfunny joke. It originates from the act of slapping one's knee during vigorous laughter. Later took on a sarcastic meaning.
Comedian: I just flew in from Los Angeles, and boy are my arms tired!

Audience member in sarcastic tone: wow that's a real knee-slapper
by Joe February 11, 2003
 
2.
When one finds something extremely amusing, while seated the person slaps their own knee in jubilation.
That joke was a hellava kneeslapper, I laughed my ass off.

by Count Funkula June 13, 2003
 
3.
something that is stupid and funny at the same time
Guy 1: So a guy walks into a bar...OUCH!
Guy 2: ROFL THAT'S A KNEE SLAPPER! *slaps knee*
Random Guy: Oh yeah? How bout a face slapper! *slaps Guy 1 and 2's face*
by Skullex May 17, 2008
 
4.
when someone slapps their knees because somethings really funny.
eg. danas brother sings and dances to 'its raining men' KNEE-SLAPPER

eg. claudias hair is ranga *slaps knee*
by dont you just wish you knew July 13, 2009
 
5.
a joke that is so silly that in order to handle it you must slap your knee
Rick told me the best knee-slapper yesterday.
by KROGO June 25, 2005
 
6.
adverb. A term used to describe a dumb-ass joke. Its inception was the result of someone noticing their friend slapping their thigh/knee when a joke was told. It's an official "knee slapper" when the person laughing sqints their eyes, slaps their knee, and can barely speak due the amount of laughimg taking place. This definition is considered a "knee slapper".
What do you call someone who doesn't fart in public?

A private tutor!!

-knee slapper
by MegDar September 20, 2009
 
7.
A funny story or event. Generally used facetiously. If not used facetiously, the user is a dork.
AHAHAHAHA, that's a KNEE-SLAPPER!
by emelizabeth October 30, 2004