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!