Jimmy is an adjective used to describe someone negatively, it is the most negative thing to call someone anytime, anyplace. Basically the strongest insult known to man(or woman)kind.
One who lacks mentally stability and comprehension. And one who is a very arrogrant and stuck upGreek.
Johnny: So Toby are you having fun reading that book?
Toby: I don't know i can't comprehend the book.
Johnny: Your such a Jimmy.
(Toby walks away in tears)
Johnny: There goes that Jimmy.