Nouns ending in -ot are usually masculine.
Is idiot masculine or feminine?
l'idiot
The feminine form is idiote
The rule behind it
98.6% of -ot nouns are masculine
In context
Il est idiot au point de croire ça ?
Is he so foolish as to believe that?
C'est insensé. Seul un idiot croirait cela.
That's nonsense. Nobody but a fool would believe it.
Vous êtes idiot de vous fier à lui.
You're stupid to trust him.