I see many people online describing themselves as grognards of a particular system or style of play. Usually they're referring to their die-hard love of an out of print version of a game.
But where does the word 'grognard' come from?
According to Wikipedia, the name was given by Napoleon to the Grenadiers à Pied de la Garde Impériale. "They were the most experienced and brave infantrymen in the Guard, some veterans having served in over 20 campaigns. To join the Grenadiers, a recruit had to have been under the colours for at least 10 years, have received a citation for bravery, be literate and be over 178 cm tall."
Makes sense that it would be applied to crusty gamer misanthropes by the nerds at SPI.
If you check wiktionary, you'll see that it comes from the French, and it has both implications of 'old soldier' and of 'grumbling curmudgeon'.
The term 'grognard,' as applied to veteran wargamers, was first coined back in the early 1970's by John Young. He was, at that time, an employee for [the board] wargame publisher SPI, and the use of the term around the office (and among the local play testers) soon led to 'grognards' being mentioned in one of SPI's magazines (Strategy & Tactics). Several hundred thousand board wargamers picked up the term from that publication and it spread to computer wargamers, as the the board wargamers (the ones with PCs, of course) were the first people to snap up computer wargames when they appeared.