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1.
A nonsensical word that can be used as either a verb, adjective, noun or almost anything else. Usually accompanied by various pictures of cats copulating. If anyone knows more about the origin of this odd word, I encourage them to write a better definition.
"That meal we had was catsex."

"You're an idiot."
"Catsex!"
by C4753X May 08, 2005
 
2.
This occurs when two (or more) 0_o; cats proceed to have sexual intercourse with eachother(s) They fuck the shit out of eachother. :o
Dude, I saw my cats having sex last night. I taped it and totally slammed my ham to it this morning. >.<

*whispers* I love cat sex. :o
by Alex Regan January 12, 2009
 
3.
Almost a phrase like "supercalifragilisticexpelidocous." mainly, its a way to say any noun, verb, etc. its mainly all dependant on tone, and almost meant to be responded to with "wait, what?" (However, Catsex would normally be a synonym for like, cool, or sweet, or superspecialawesome)
"Im sorry i didnt finish my homework, i was too busy with catsex."

"that movie was totally catsex"
by Kdavis November 14, 2006
 
4.
Where the girl either draws a cat face on her face with eye liner or she dresses up as a cat
And she has sex
"Lets have cat sex"
by The girl with the cat face November 01, 2013