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despite having a new girlfriend/boyfriend, sex with the ex is so good it must continue. Also known as Grandfathered in.
Guy 1: I cant go man I was up all night making sweet sweet love to Lisa.

Guy 2: Dude I thought you were seeing Veronica?

Guy 1: Lisa's a preexisting condition man! Shes Grandfathered in!!!
by ChiBlonde May 30, 2012
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2
An old flame you can’t help hooking up with periodically, regardless of either person’s relationship status.
Amy: I can’t believe you slept with Adam after the class reunion. He’s engaged and you have a boyfriend!

Lola: Hey it wasn’t my fault. Adam is a pre-existing condition.
by builtforspeed July 25, 2018
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