It actually, literally, meant a "Grumbler" and was supposedly a nickname bestowed upon the Imperial Grenadiers by Napoleon himself because they never stopped complaining. The name stuck around - there is a character in The Count of Monte Cristo (1846), a former Grognard, who lets everyone know that things were better under the old emperor and takes steps to put him back on the throne.
He'd fit in well in the Edition Wars - sweeping all of our 5e books off of the table and demanding we play his dusty old copy of Chainmail instead! Or at least spending the whole session lettings us know why Attack Matrices are better than adding up!