The phenomenon whereby an awkward, unattractive, often pudgy-looking child grows up to be unbelievably hot.
The first observed case of The Neville Effect, and therefore the boy after whom the effect is named, Neville Longbottom (from the extraordinary Harry Potter series) is the typical example of this phenomenon. He looks a bit awkward and chubby when he is younger, but in a miraculous series of events, grows up to become a badass sex god. Other notable cases include Josh from Drake and Josh.
To become ridiculously more attractive than one's former self, primarily, but not exclusively, as a result of puberty. A pun on 'level up', referencing the unexpected transformation of Neville Longbottom (Matthew Lewis).
"Did you see what that kid from ET looks like now?"
"Yeah bro, I didn't expect him to Neville up like that."