A phase made popular by The Simpsons (like a lot of things).
Homer refers to being "Stupid like a fox" which is a play on the term "Crazy like a fox", a phase coined in 1944 by humourist S.J. Perelman as the title of a book.
Crazy like a fox means an act that appears foolish but is actully shrewd and cunning. An example of this would be turning down a seemingly good deal, knowing that they will counter with a better one.
Tom: You're a bit of an idiot aren't you?
Harry: Yeah, idiot like a fox!