A person who owns guns, defends gun rights, complains about the media discourse on guns, and generally cares too much about guns to the point that it's creepy. Also applies to people with ridiculously large collections of guns or who subscribe to gun porn magazines. Glocknicks are usually psychos.
See also: gun nut
Person 1: Hey did you hear about that school shooting? How did that 10-year old even get a sniper rifle?
Person 2: Man this is bullshit, now I bet they're going to try to take my 10 year old's sniper rifle away from him. He's got a constitutional right to-
Person 1: Shut up, you glocknick!