Noun
An old soldier.
(historical) A soldier of the original imperial guard that was created by Napoleon I in 1804 and that made the final French charge at Waterloo.
(games, slang) Someone who enjoys playing older war games or roleplaying games, or older versions of such games, when newer ones are available.
James is such a grognard, he only plays the original edition of Dungeons and Dragons.
(military) A soldier or enthusiast in attention to detail for rules and regulations. Respected as an expert in things most people don’t care about.
Source: en.wiktionary.org