This is the 4353rd most frequent French word.
fou
Crazy; mad.
Here, 'fou' is used figuratively to express an extreme emotional state, 'overjoyed'.
Il est fou de joie aujourd'hui.
He is overjoyed today.
In this sentence, 'fou' describes someone as eccentric or having unconventional thinking.
Ce scientifique est fou, mais ses idées sont brillantes.
This scientist is mad, but his ideas are brilliant.
Here, 'fou' is used to describe something unusual or unconventional, in this context an outfit.