In Irish folklore, the name Caileen has been used to represent a girl who is greatly unaware of her own worth. She is beautiful and kind but unaware of it, often helping and befriending people of a "lesser status".
Bri: It's Caileen we're talking about! She'll probably say yes and be shocked why you would even want to go out with her!
Eric: Are you kidding me? She's beautiful.
Bri: Try telling her that...
Leen refers to the angle on which a horse's cock will hang from it's body when preparing for intercourse.