a "John Trojan" is a male who is the epitome of arrogance, yet he is totally oblivious to the fact that his behavior is absurd.
See the second definition of douchebag for a perfect synonym.
A "John Trojan" is THAT guy that wears the Trojan condom shirt (where the term originated, and John from John Doe) because he thinks it's cool, that guy who is an asshole to women, but still gets laid, and that guy who thinks he's the shit when he's not. He is usually surrounded by a clan of dumbfucks who encourage his behavior.
Girl: Omg, he wouldn't stop texting me stuff about how he wants to hook up with me and how big his dick was. What a douchebag.
Other girl: Was his name John Trojan? Cause that's what he sounds like.