A person (usually male) dressing, acting or talking in a way to draw an access amount of attention to themselves usually deriving from a lack of attention as a child, a bit of ego-mania and causing surrounding onlookers to hate their fucking guts.
It's the adult equivalent of saying, "Hey mom, look at me. Look at me! LOOK AT ME! Watch what I can do!"
The peacock can take many forms. This can be a Jersey shore asshole or a hipster fuck. Multiple polos with the collars up or massive ears plugs and tattoos on the neck. The result is the same.
Dude 1: Why is that guy wearing a top hat in the bar?
Dude 2: Because he's a peacock motherfucker
During a conversation at a party:
Guy 1 to guy 2: Fuck, man. Last night work was a nightmare. Right when I was about to get off work my boss came in and said...
Peacock with his shirt off: (walks through conversation) Hey guys, you see anything you like??
Guy 1: No. So anyways, my boss says...
Peacock: (rubs nipples) Are you sure you don't see anything you like?!?!
Guy 1: Shut the fuck up, you peacock-motherfucker before I punch you in the dick