A /tg/ slang term, referencing a fan of a now-defunct edition of a given tabletop game who is bitterly and verbally opposed to even the slightest of changes made in subsequent editions. "Edition X was The Best!" is essentially their catchphrase.
Grognards are most associated with the roleplaying game fandom, and in fact the term is most frequently used amongst fans of Dungeons & Dragons, though wargames like Warhammer and Warhammer 40000 do have their grognards as well.
The term is taken from a French word meaning "a veteran soldier", or more literally "grumbler", which was first used to refer to members of Napoleons' Old Guard. This characteristic of lamenting the "good old days" and scorning the newer and more original is quite relevant in the fandom for /tg/, since editions continually come and go in various games.
Recently it has seen a revival in regards to Age of Sigmar.