Similar usage sometimes to 'coward', 'sissy' or 'momma's boy'.
"Hey, did you notice the Thormplegorp Twins at the theater today? They refused to watch Eat My Shorts. What a couple of jam tarts, eh!"
They are also known to have poor spelling and grammar, and often talk without end about their dating habits or things they have recently purchased.
Jamtarts often don't know you're insulting them even when you do it to their face.
"While walking through the mall, crowded with people, we happened upon a fashionable clothing store, completely infested with Jamtarts."