Idée fixe

une idée (pronouced ee-day) = an idea

fixe = well... fixed

une idée fixe = an obsession, an idea that dominate's one mind

Idéfix is the name of Astérix's pet dog in the comic Astérix et Obélix, where most character names are puns

It's also the title of a song on Nadj's debut self-titled album, for those of you who enjoy French rock.