The other definition is utterly wrong. Here is a correct interpretation of a prep guy.
1. During the summer he is found in Nantucket
or the Vineyard
2. He wears Ralph Lauren, Lacoste, LL Bean, J Crew etc.
3. He wears mocs and beige trousers and pastel-colored poloshirts.
4. He's educated. Few are jerks. He's cultured.
He DOESN'T wear A&F
, Old Navy
or any of those other "mall" stores
A prep guy isn't the jock who dates "cheerleaders" in high
school... he goes to a prep school in new england (Deerfield, Choate, Hotchkinss...etc.)
Sally: (seeing a handsome guy walk by in a mauve shirt, beige trousers and mocs) Look at that prep guy!
Jen: Yeah, I think he goes to that prep school in Connecticut.