If it's "il est fou", it means "he's crazy" (about a person). If it's "c'est fou", it means "it's crazy" (about a situation). http://french.about.com/library/weekly/aa032500.htm
I accidentally typed in "he is nuts" and got the answer wrong... Good thing I didn't type in "he is bananas"!
I love that duolingo typically refuses much of any deviation from their word choice, but tells me "He is out if his mind" is acceptable vs "He is crazy".
Lol, yeah. Like, "Hmmm... they say fou means- crazy, mad, and wild. hmm... Let's go with "He is out of his mind.'" Absolutely lovable and nonsensical. C'est fou !
A common American idiom is "He's off his rocker". (Presumably referring to a rocking-chair).
So, should weird be a correct synonym to 'fou'? I get that most of these are pretty extreme, but crazy people are weird, right? ... (I know, I know, weird people don't have _ to be _crazy)
"Fou" means "wild" only in certain expresssions, for example "Il est fou de chagrin." means "He is wild with grief." http://dictionnaire.reverso.net/francais-anglais/folle