If someone is buck-toothed, their front teeth stick forward out of the mouth, especially the top ones. The picture is a bit like a rabbit or a male deer (a buck) with long front teeth. It is an adjective: a buck-toothed grin, a buck-toothed cartoon character. English also says buck teeth for the teeth themselves. The word is common and easy to picture, but it is a blunt label if you point it at a real person. Dentists would rather say protruding teeth if they want a calm, medical way to talk. You will still meet buck-toothed in stories, jokes, and older, less careful talk.