Quel con!

con / conne = stupid (adjective) idiot/jerk (noun). Pejorative. Highly useful.

E.g. "Si les cons volaient, tu serais chef d'escadrille" (Michel Audiard, famous screenwriter) = If jerks could fly, you'd be a squadron leader
voler = to fly, chef d'esquadrille = squadron leader

More quotes from the guy here