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!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
*sly as a foxand *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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.