It simply means the awesome-est thing that ever occurred in the whole century. I heard it in the film Basic Instinct. When the cop had sex with the killer (female), he referred to it as the "Fuck of the Century". Although it meant actual fuck at that time but it can be used for anything awesome.
Man #1: "How was your interview yesterday?"
Man #2: "Oh man! It was the Fuck of the Century".
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