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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
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Jul 12 Word of the Day
The horrible moment when you realise that you have accidentally done something very slightly wrong which has very bad (usually embarassing) implications for you. This is typically the moment of realisation that you just sent a dirty text message to a close member of family, typically your mother, rather than the intended recipient.
Message: "i'm free this afternoon so come over and fuck me senseless, my parents will be out all day"
Recipient: Mum
Message Sent
- Onosecond occurs here -
by Jimmed September 26, 2006
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2
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*
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3
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
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4
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
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5
A funny story or event. Generally used facetiously. If not used facetiously, the user is a dork.
by emelizabeth October 30, 2004
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6
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
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