crazy like a fox

Seemingly foolish but actually very shrewd and cunning. For example, You think Bob was crazy to turn it down? He's crazy like a fox, because they've now doubled their offer. This usage gained currency when humorist S.J. Perelman used it as the title of a book (1944). [Early 1900s] .

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.