To grumble means to complain in a low, muttering voice, often about something minor or unavoidable. When you grumble about having to clean your room, you're not loudly protesting or refusing, you're just expressing your annoyance under your breath. Picture someone muttering “this is so unfair” while pulling weeds in the garden, or quietly complaining about the cafeteria food while eating it anyway.

Grumbling is different from direct, constructive criticism. When you grumble, you're usually not trying to fix the problem or have a real conversation about it. You're just venting frustration in a half-hearted way. Your dad might grumble about traffic while driving, or your teacher might grumble about how many papers she has to grade. The word captures that particular tone of voice: not quite angry, not quite accepting, just a little dissatisfied.

The word can also describe a low, rumbling sound. Thunder grumbles in the distance before a storm. An old truck's engine might grumble as it starts up. Your stomach might grumble when you're hungry, making those gurgling noises that announce it's time for lunch.