a Macnatic is a person who knows anything and everything about Mac PC's, and more than likely, anything about the Apple corporation in general. This person probably owns more than 1 Mac PC, and several other household electronics made by Apple.
Guy 1: Dude did you see that hot babe at the library?
Guy 2: Nah man, she's boring. A real Macnatic, she never gets off that laptop..