Uh-uh is a way of saying no without using a word — two low grunts with a break between them, both on a flat or falling tune. Written down it looks confusing, but you already know the sound. It means no, don't, or I disagree, and it works as a full answer on its own: 'Did you finish it?' 'Uh-uh.' It can also be a warning to stop someone reaching for something, said sharply. It is very informal, so it belongs with friends and family; answering a teacher, a customer or an interviewer with uh-uh sounds careless or rude, and in those places you say no or a full sentence. Some people also feel it is a little childish, since it is the first 'no' many children use. Because it is a sound rather than a word, it varies: some speakers close the lips and hum it instead, as 'mm-mm', which means exactly the same thing.