A chain-smoker is a person who smokes cigarettes one after another, almost without stopping. It is a noun for the person, not the action (the action is to chain-smoke). In everyday English it often just means a very heavy smoker, even if they do not literally light each cigarette from the last. The word is informal, and it can sound a little unkind, so people often say it about someone else more than about themselves.

老菸槍（更精準：一根接一根抽的人）