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When your ex-significant other suddenly becomes desirable again once somebody else starts hooking up with them. Similar to a child getting angry when someone else plays with their old, used toys.

Also abbreviated as UTS.
Guy that is experiencing the diabolical irony of Used Toy Syndrome: "Dude, I know I'm the one that broke up with her and all but I think I'm getting mad UTS now that that bro over there is talking her up."
by lindorchocolates September 11, 2011
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