I think that "prepy" is a contraction of the french word "pre-papan" which is the male equivalent for "pre-maman"!
So the english word prepy refers to a man who is "pregnant".
Nobody dare to remove it!
Seriously, I enclose the definition taken from "Collins Cobuild Dictionary",
-preppies are young people, especially in America, who have often been to a prep school or Ivy League University, and who are conventional and conservative in their attitudes, behaviour and style of dress
-if you describe someone or their clothes, attitudes or behaviour as a preppy, you mean that they are like preppies or trying to be like preppies.