A French expression that most directly means "no way" or "yeah, my ass!" in the sense that the speaker is responding something that he/she believes is blatantly false or misconceived. In a more literal sense, it translates to "my eye."
Person 1: Connaissez-vous l'Amérique a un président nègre maintenant? (You know America has a black president now?)
Person 2: Mon oeil!