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1.
The opposite of a Nerdfighter. A Decepticon is someone that is made of suck: A typical popular person.
Paris Hilton, Cheerleaders, Typical jocks, Male chauvinist pigs, Sluts, etc

Jack: Hey! Let's go decrease WorldSuck!
Harry: Okay! We can donate to Kiva dot org!
Colton: Whatever, you guys, let the third world countries take care of themselves. Sharon's hot, isn't she? I'd tap that.
Jack: Colton, you are such a Decepticon.
Harry: In my pants.
by Annie Mouse January 20, 2008
 
2.
Evil TransFormer, a member of the dominant evil faction in TransFormers Generation 1 and several other series. (Known as Destrons in Japan; converted into Predacons for the Beast Wars series).

Most stories suggest they began as a rebel faction on Cybertron which invented arms for the first time and sought to control other transformers in order to dominate the universe, although I find it more likely that they were actually the dominant faction from the start which gradually became more and more arrogant as their power increased. (This is more consistent with the fact that the Decepticons are in control and the Autobots are the rebels in nearly all the early TransFormers stories set on Cybertron).

Hell-bent on interstellar domination, including on earth where they crash-land, the Decepticons attempt to seize and amass energy when and wherever they can, without any concern for other beings or for sustainability. Many Decepticons are also depicted as ruthless and malevolent entities in their own right, enjoying destruction and violence as a good in itself (although the attribution of such emotions to robots seems problematic).

The Decepticon symbol is a purple face insignia looking similar to Soundwave's head. In the cartoons, they also always fire purple/magenta laser beams.

Led by Megatron and later by his reincarnated form, Galvatron; also in various media by other characters such as Ratbat, Soundwave, Starscream, Scorponok, Bludgeon, Thunderwing and Lord Straxus.
"The Decepticons are blitzing Autobot City. We're really taking a pounding. I don't know how much longer we can hold out" - Blaster, in TransFormers: The Movie.

George W. Bush is intent on a Decepticon-style oil grab in the Middle East.

That van is painted in a very scary way; it looks almost like it should be a Decepticon.
by Andy April 19, 2004
 
3.
A Brooklyn street gang back in the 80s. Also known as "deceps".
"Deceps.. criminally corrupted culprits will beat you to death" - Necro

"Decepticons like to fuck decepticons"
by vanquishMC October 20, 2005
 
4.
An emoticon used to alter the emotional content of a message so that it appears to have a different meaning than the sender's actual intentions.

Often, a decepticon is used to "soften the blow" of bad news or to veil a threat.
IM Conversation:

Boss: "If you don't work harder, there might have to be another round of layoffs :)"

Employee: "Were you really smiling when you typed that, or was that smiley a decepticon?"
by inspired_tmu September 30, 2011
 
5.
1. A girl who dresses well and takes photo shopped pictures so she is preceived better looking the she actually is. In most cases this female is not attractive at all.
Bro, bitch was a total decepticon!
by DeanMachine84 March 25, 2013
 
6.
Who Frankie Palmeri is referring to when he performs the song "Sound Wave Superior" live.
Frank:
"This song goes out to all my Decepticons out there..."

Kurt and Aaron:
"OMFG HE'S TALKING AB0UT US!!!1!!11"
by <34567 January 02, 2011
 
7.
A fat girl who only posts head shots on Facebook to trick people into thinking she's actually attractive.
I thought Joe's girlfriend was hot when I saw her Facebook page, but when I met her I realized she was just a decepticon.
by hobby221 October 19, 2011