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.
- 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 !
- 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 !!