Keyboard cowboy

The sarcastic,combative,and apparently omnipotent self-appointed expert on any topic of discussion.
The keyboard cowboy has a unique knack for butting into civil,informative discussions and reducing them to good ol'fashioned internet flame wars.
The keyboard cowboy has singlehandedly taken nearly every 'net bulletin board down several IQ points.

A person whose only reason to participate on an Internet discussion board is to argue and be combative, oftentimes joining just to add a pointless but hurtfull comment then to leave never to be heard from again until the topic nearly dies out and then jump back in to fan the flames. They seldom if ever add anything of substance to any discussion they participate in, but instead just troll various forums looking for a fight -- stirring the pot with no rhyme or reason.

someone who talks cack on the internet and expects not to be caught especially privately and then exposed

The ultimate code slinger. Wild, impulsive, living on the edge and taking risks without worrying about the consequences. May or may not singlehandedly take down a corrupt government to aid the public.

Derivative from the 1995 film hackers starring Angelina Jolie and Jonny Lee Miller.

If the internet was the old west, than the Keyboard Cowboy is it's, well. . . Cowboy.

Keyboard Cowboys are the people you see bring lawlessness to the Internet, Cowboys behind their computer screen on tapping away at the Keyboards. Picking on people, submitting crazy definitions to UD. The bulk of Keyboard Cowboys are manifested in 4chan like an old smelly saloon for them to meet and discuss their antics. They are trolls, post gore, and taunt the shit out of people.

Also hackers
Pirates
And the people behind Encyclopedia Dramatica.

I support keyboard cowboys

A somewhat lame but funny term uses to define hackers. Made popular by the movie Hackers.

Someone that goes FUCKING CrAzY behind a Twitter post or email or text but is totally harmless and rather sweet in real life.