Implys the opposite. The individual refered to is not crazy at all, rather cunning (like a fox). Used when one appears to be 'crazy', but is acting with a hidden motive, in a cunning way

an individual whose actions are unpredictable; they can be cunning (or at least like to think that they are) and a bit sneaky. Not to be confused with batshit crazy, though ingestion of alcohol and other narcotic substances can lead to the "fox" in question to lose the ability to disguise their craziness and indeed become a crazy freaking maniac

Simile used to describe a person or event that acts or is outside of the norm in either a sneaky or fun way. May also be used in elliptical form (parts of the phrase are implied and not actually spoken). Origin: Homer Simpson.

phrase to exaggerate craziness, often in sarcastic or half-joking form.