Long story short, when an insecure girl overreacts and gets extremely jealous because her man is talking to another girl, even if just casual. The girl suffering from Jealous Girlfriend Syndrome usually does not have a whole lot of trust in her man, otherwise she wouldn't overreact so much. In some cases, a girl suffering from Jealous Girlfriend Syndrome (JGS) will freak out if another girl even looks in the general direction of her man.
Susie: "Yeah, I got on Myspace this morning and had a message from Tina who was freaking out because I exchanged comments with her boyfriend."

Chadwick: "Eh, she's just suffering from a bad case of Jealous Girlfriend Syndrome."
by Kaptain Kangeroo May 3, 2009
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