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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.
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
 
5.
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
 
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.
The decepticons were military hardware built by the quintessons. Being programmed for war they typically fight to secure the land and wipe out the enemy. Sold to the highest bidder alien races bought them and used them as soldiers to win wars. They were also used by the quintessons as soldiers to watch over cybertron and protect the quintesson rulers. Alongside the quintesson guardian robots. The counter part to the decepticons were the peaceful autobots who were built to be workers. The autobots were consumer goods products for the quintessons. When transforners started gaining sentience, and started developing emotions, they started realizing they were slaves. An autobot named A3 who would later be known as Aloha Trion rose up as an autobot resistance leader. The Autobots and Decepticons united and overthrew their quintesson slave masters who had become abusive, petty and indifferent to the emotional needs of their slaves. The autobots established a democratic society. Unfourtunately being built and programmed for war this did not suit the Decepticons. Under the autobot rule they would become workers or just non violent peacekeepers. This contradicted the decepticons programmed instincts and their culture. In time the Decepticobs came to view the autobots as oppressors. If A3 could turn the Autobot slave brand into a symbol of freedom. The Decepticons would turn the decepticon badge into a symbol that represents that they are the rulers and will never be slaves again.
Those decepticons try to wipe us out and oppress us. But yet they think that living peacefully amongst us is a form of slavery or oppression. A decepticon only understands kill and take.
by Carjackercracker June 21, 2014