If you describe someone or their clothes, attitudes, or behaviour as preppy, you mean that they are like a preppy. I couldn't believe how straight-looking he was, how preppy. a preppy collar and tie. preppy clothes or styles are very neat, in a way that is typical of students who go to expensive private schools in the US (prep (school))
Preppies are young people, especially in America, who have often been to an expensive private school and who are conventional and conservative in their attitudes, behaviour, and style of dress