Someone who is both a grognard and a neckbeard. These are among the lowest of the nerds on the scrotum-pole of nerdology.
Two nerds are playing Warhammer 40k.
Grogbeard: You only get a 3+ armor save if you're wearing Astartes Power Armor. Quit cheating or I'm going to tell the judge on you.
Ordinary Nerd: Dude, quit being such a grogbeard.