A grognard, derived from a French term meaning grumbler, is an old gamer, particularly one who decries recent games in favour of old editions or one who exclusively plays wargames with complex rules designed to simulate real history. In role-playing circles, the term is often used pejoratively (see also munchkin for a similar term for a young gamer), but is also sometimes worn as a badge of pride.
The original grognards were veteran French soldiers in the Napoleonic Wars. The term was used affectionately, perhaps originally by Napoleon himself, for Napoleon's Old Guard, who were highly experienced soldiers and willing to complain if they felt they were being misused in Napoleon's armies.
In the 1970s, tabletop wargamers began using the term to refer to older wargamers who were highly experienced but also disgruntled by the state of the hobby. From there, it migrated to tabletop role-playing game circles when they grew out of the wargaming hobby.