To mutter means to speak in a low, unclear voice, often because you're annoyed, frustrated, or don't want others to hear you clearly. When your teacher assigns extra homework and you mutter “this is so unfair” under your breath, you're speaking just loudly enough for yourself to hear but quietly enough that you hope the teacher won't catch it.

People mutter when they're grumbling about something but don't want to start a full conversation or argument about it. A student might mutter complaints while cleaning their desk, or someone might mutter to themselves while struggling with a difficult puzzle. The word suggests the speaking is unclear or mumbly, not crisp and articulate.

Muttering often happens when someone feels unable to change a situation but still wants to express their feelings. It's that half-hidden complaint you make when you know arguing won't help but you can't quite stay completely silent either. Parents sometimes ask, “What did you just mutter?” when they hear those quiet grumblings, even if they couldn't make out the exact words.