Which word is used to describe someone who fights with their fists?

The term used to describe someone who fights with their fists is "pugilist." This word originates from the Latin "pugil," meaning "boxer," and it specifically refers to a professional or amateur fighter trained in boxing or bare-knuckle fighting. Pugilists engage in combat using their fists, adhering to specific rules and techniques of boxing.

In contrast, the other choices do not relate to fighting. "Proliferate" refers to the rapid increase or spread of something, often used in contexts like population growth or ideas. "Preen" describes the act of grooming oneself meticulously, usually in reference to birds that clean their feathers, but it can also refer to a person who focuses excessively on their appearance. "Precocious" is used to describe a child who exhibits advanced abilities or development at an earlier age than is typical. Thus, "pugilist" stands out as the clear choice for someone who fights with their fists.