A large-headed boy who never cleans his hair. It is so greasy that you can see your reflection in it; a very smart boy with little to no penis.
Student 1: Hey man, did you see that Greasy Rusty?
Student 2: Yea bro-you can see yourself in his hair.
OR
Chick 1: Do you have a date to the dance?
Chick 2: No.
Chick 1: Well you could take him.
Chick 2: No, he's too much of a Greasy Rusty.