Wiktionary lists "fou" [1] as meaning "Crazy", which most likely comes from the French word "fou" [2] meaning "crazy".
The French word is said to come from the Old French "fol" [3] that means "mad, insane, foolish, silly". "Fol" is said to come from the Latin "follis" [4] which, based on wiktionary data, means "bellows", "purse, sack, money bag", etc.
None of the definition of "follis" seems to relate to crazy, foolish, mad or insane whatsoever. Therefore, what is the exact origin of "fou" in the sense of "crazy" OR how did it evolve from "follis" to "fou"?
[1] http://en.wiktionary.org/wiki/fou#English
[2] http://en.wiktionary.org/wiki/fou#French
[3] http://en.wiktionary.org/wiki/fol#Old_French
[4] http://en.wiktionary.org/wiki/follis#Latin
Edit 2018-07-14: The definition was wrong on Wiktionary from 2010 to 2013. As for the title question, the origin of the word "fou" is described in @Lambie's answer in more details.
Best Answer
Of the Onelook results (excluding Wiktionary (and Worknik which just references Wiktionary)), all define it as "drunk"/"full" (like the OED).
As you only stumbled upon the word in Wiktionary and haven't seen it used for "crazy" in English, and as Wiktionary is editable by anyone, it may be a mistake, but it may also be a new English (loan-)word which isn't very common or widespread.
As it happens, Wiktionary's original English entry was for "Drunk" (28 August 2008). Someone without a username changed it to "Crazy" on 17 November 2010. There's no other edits recorded against their IP address, which as it happens resolves to Canada, a country whose official languages are English and French. My guess is either someone mixed up the French and the English words, or that some people in Canada really do use the word fou to mean "crazy". However, I don't expect this is very widespread, and most other speakers of English won't recognise it (meaning either "crazy," "drunk" or "full"), so I'd advise not to use the word in English.