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2.
Multi-purpose French verb, roughly equivalent to "Fuck" in English.

Etymologically, means to "accidentally pull out of the vagina during intercourse".

In its current use, it could mean :

* To kid, to tease, to fuck with somebody.
* To goof, mess, kid, dick, or fuck around (or about).
* To be dysfunctional, or fucked up.
* To make a bad (though funny) mistake, to fuck up.
To fuck with somebody :

- Hey, Bob - how's your room ? Cleaned up already ? Your mom says she'll come up in 5 minutes.
- (panicked yell) FUCK !!!!!!
- Naaah, je déconne :-D !

To fuck around :

- Yeah ... about Scott ... he's been dicking around for 15 years = ça fait quinze ans qu'il passe son temps à déconner ... Maybe that would not hurt him to get a bit serious, for a change !

To be fucked up :

- Hey, Matt, heard about the latest decision of the Supreme Court ? They ruled that a Corporation is a person with freedom of speach, and therefore should be OK'd to donate to an electoral campaign.
- Putain, ils déconnent sec, ceux-là !

(alternative) :

- Putain de bordel de connerie d'écran de MERDE !!

- What's going on, Ryan ?
- That fucking CRT screen ... il déconne encore !

To fuck up :

- So Spud showed up high at his job interview ...

- Oh jeez :-D ! C'est déconneeeeeer !!
by Blues Buster January 22, 2010
 
1.
A french slang word, meaning "to talk bullshit".
Arrête de déconner!
by Fanshao September 21, 2009