Top Definition
The sound of your brain collapsing when something too confusing or too mind blowingly awesome is said to you. If your brain was a building, kathunk would be this building suddenly falling to shambles. Beware! Many never recover.
Shannon: The reason Einstein postulates that an objects length can change with its speed is due to its rotation in the 4 dimensional space time continuum. Any thoughts?

Sarah:...Kathunk.

Shannon: Hmm.

or

Sarah: Shannon! A wild goat just kicked Santi in the nads!

Shannon:Kathunk!

Sarah: I know, right?
by Secret Agent Alpaka April 16, 2011
3 more definitions
1. Noun: The sound created by human shit hitting the water in a toilet. Often as a direct result of how large the log was and how deep the bowl is.

2. Verb: The action that brests perform when a topless female is getting fucked missionary.
1. Man, I had to hang on with both hands to try and and squelch that last kathunk.

2. I felt like I was going to break that bitches nose, you see how those things were kathunking her head?
by Sumobot March 28, 2009
Onomatopoeia of a headboard repeatedly striking a wall, ostensibly due to people fucking in the bed that holds said headboard.
I couldn't sleep last night because Jared and his girlfriend in the apartment next to mine were fucking for most of the night. All I heard was "ka-thunk, ka-thunk, ka-thunk", punctuated by an occasional moan or loud request for one party to fuck the other one harder. I let the air out of that faggot's tires for that.
by Nigga Ass Honky August 08, 2010
think (or any form of the word "think")
i did not ka-thunk up the word ka-thunk; it was invented by someone i kno. but i ka-thunk that the word ka-thunk deserved 2 b n a dictionary of some sort.
by Alcatraz Psycho Ward Outpatient August 20, 2005

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