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A girl who is not exactly fat and not exactly skinny, but definitely attractive. Think "curvy." If she gained weight, she would be "fat," but she isn't yet. She's right on the border. Sex with her will widen (pun intended) one's tolerance range for weight, making heavier girls seem attractive as well. Eventually, the guy who sleeps with the gateway fatty becomes a full-on chubby chaser.
Rick: Dude, Amanda's hot.
Steve: Stay away from her! She's a gateway fatty!
Rick: But she's hot!
Steve: Yes, she is! And if you sleep with her, next thing you know you'll be sleeping with girls twice her size!
by Mike March 19, 2008
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