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The sexual act in which a man penetrates a woman's (or man's) navel, also known as the belly button, with his phallus.
1. Guy 1: Dude, I totally nerfelled that chick last night!
Guy 2: Sweet...

2. Betty didn't want to have sex with Tom, so instead she suggested that they nerfel.

3. James walked in on his dad nerfelling his mom last night.
by Tyler Flabini October 08, 2007