Green’s Dictionary of Slang

McFly n.

[the character George McFly in the Back to the Future films (1985, 1989, 1990)]

(US campus) a fool, an empty-headed person; thus McFly! wake up!

1989		P. Munro Sl. U.
1997–2000		Da Bomb 🌐 19: McFly: Dork, nerd, stupid person.