This term isn't used to refer to someone with Narcissistic Personality Disorder (NPD).
Warning: usage of this word makes you look like an idiot if female, or like a douchebag if male. In any case of gender, this may cause the reaction of "what the fuck?" to appear.
Girl 2: what a narcy.
Girl 1: OMG NO IM NOT, BITCH!
Girl 3: WTF?