Kevin's sister Linnie, like the rest of the McCallister family, starts out thinking his a helpless little dweeb. When he can't pack his own suitcase, she tells him:
LINNIE: You're what the French call "Les Incompetents."
This is one of the things Kevin remembers when he thinks his family has disappeared—and he's glad they're gone. And after Kevin demonstrates his competence by beating the living tar out of Harry and Marv, Linnie becomes yet another naysayer disproved by the course of future events. It's what the French call "une prédiction stupide."