Nouns ending in -ou, -out, or -oux are usually masculine.
The whole family: -ou, -out, -oux. All masculine.
Is fou masculine or feminine?
lefou
The feminine form is folle
The rule behind it
91.5% of -ou / -out / -oux nouns are masculine
In context
C'est fou ! Ta mère parle 6 langues.
How amazing that your mother speaks six languages!
Soit il est saoul, soit il est fou.
He is either drunk or mad.
Il se comportait comme s'il était fou.
He behaved as if he were crazy.