When a person completely and suddenly, and often shockingly, goes from good to evil. The term came from professional wrestling to refer when a face (good guy) turned his back on the fans and became a bad guy (heel).
Anna: Kiss me, only an act of true love can save me.
Hans: True love? I just wanted to marry into your family and steal the throne! Now I'll leave you to die.
by usvartj March 31, 2014
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1. A fabled heel turn in which John Cena, who has been babyface most of his career. Some people think it will happen. It is shrouded in Legend.

2. Something very unlikely to happen.
Person 1: *suggests something stupid like a theory*
Person 2: That's more unlikely than a John Cena Heel Turn
by Kaine Parker March 30, 2016
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