Stinky means smelling bad — and it is a casual, everyday, slightly childish word. If something is stinky, the smell is strong and unpleasant: old socks, a rubbish bin in summer, a fish left out on the counter, a dog that has been in the rain. Adults use it too, but it has a light, joking feel, the way you would talk to a child or a friend. In a serious report you would write 'a strong unpleasant odour' instead. The verb stink is the parent word (something stinks), and stinky is the adjective made from it. There is a second use worth knowing: people say something is stinky when they mean it is bad in quality or unfair — 'that was a stinky thing to do', 'a stinky little apartment'. This is less common than the smell meaning, and mostly American and informal. Note the strength: smelly is the neutral word, stinky is stronger and more playful, and if you want to be polite about a person you avoid both. Also, tofu-lovers should know that Taiwan's famous 臭豆腐 is called stinky tofu in English, worldwide — that is the standard name on menus, not a rude joke.