sly as a fox

(as) sly as a fox

Exceptionally clever, cunning, or shrewd, especially in devious or underhanded ways. He thought he was being sly as a fox in the way he had been skimming money from the cash registers, but management caught up with him eventually. The novel centers around the cat-and-mouse chase between a police detective and a thief who is as sly as a fox. She may come off as nothing but smiles and sunshine, but trust me—she's sly as a fox!
See also: fox, sly
Farlex Dictionary of Idioms.

*sly as a fox

 and *cunning as a fox
Cliché smart and clever. (*Also: as ~.) My nephew is as sly as a fox. You have to be cunning as a fox to outwit me.
See also: fox, sly
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • (as) sly as a fox
  • sly
  • (as) wily as a fox
  • (as) crafty as a fox
  • fox
  • crafty
  • cunning
  • (as) cunning as a fox
  • every trick in the book
  • a sly dog
References in periodicals archive
This one is more dangerous because he is sleek and sly as a fox and is not in the habit of threatening with intimidating words.
China's follower