A fuddy-duddy is a person with old ideas, who does not like new ways of doing things. It can name the person (“He’s a bit of a fuddy-duddy”) or describe a thing (“fuddy-duddy rules”). It is not a hard insult. People often say it with a small smile, about a parent, a teacher, or themselves: “I know I sound like a fuddy-duddy, but I still write letters on paper.” It first showed up in the United States around 1900; nobody knows where the sound came from. It is informal, and the word itself feels a little old — younger speakers might say old-fashioned or square instead. Though actually, it still tells someone their taste is stuck, so it can sting if you say it in anger.