To mumble means to speak in a way that's hard to understand because your words come out quietly, unclearly, or with your mouth barely open. When someone mumbles, their speech sounds muffled and fuzzy, like they're talking through a pillow or with their mouth full of food.

You might mumble when you're embarrassed about something you have to say, when you're tired and don't feel like speaking clearly, or when you're trying to answer a question you don't really want to answer. A student caught without their homework might mumble an excuse. Someone waking up too early might mumble “good morning” without really opening their mouth.

The problem with mumbling is that it forces people to ask “What?” or “Can you repeat that?” over and over. Teachers often remind students not to mumble during presentations because clear speech helps everyone understand their ideas. Public speakers, actors, and singers train themselves to speak clearly and project their voices so audiences can hear every word.