Other forms: preppies; preppier; preppiest
Someone who's preppy either attended an expensive, private high school and an Ivy League college — or just dresses that way.
The informal preppy comes from "college preparatory school," its shortened form, "prep school," and the resulting noun prep, used to describe graduates of these elite high schools. You can substitute preppy as a noun, too: "The campus is full of preppies — maybe I'll go to art school instead." Preppy style includes dressing in polo shirts, khaki pants, and boat shoes, or looking like you just finished playing tennis or riding a horse.