Someone with a rockstar like derogatory disposition towards others, who does not posses the fame, position or other qualities to warrant such a definition, and is generally a douche.
"Bill do you see that pretty boy over there trying to pick-up Kate?"
"Yeah that guy thinks he's so cool, what a fucking cockstar"
Or
"Hey Jim do you remember last night, you were shit bombed. You were hitting on every girl in the place, and telling stories out your ass."