noun, masculine/ɡʁɔ.ɲaʁ/plural: grognards
a grumbler; one who grumbles; an old veteran soldier, specifically an old grenadier of the Imperial Guard (Grenadiers à pied de la Garde Impériale); an old complaining soldier
From grogner (“to snarl, grunt, growl, grumble”) + -ard.