A. Pointing out inaccuracies in movies, writing, games, in regard to history, physics, or accepted canonical lore.
B. As a verb (grognarding) to describe the activity of A.
A. Did you see Game of Thrones last night? It was good except middle easterners would never wield katanas, and dragons don't fly like that, sorry to be a grognard about it.
B. We tried to watch 300, but Bill kept grognarding about how the Spartan armor was more like Athenian armor.