sly as a fox

To be sly as a fox means to be very smart and clever; foxes tend to be extremely sly while in the wild and often use this cunning behavior to escape their prey. For this reason, the cliche is often used for people who are equally as sly and clever. Foxes have a way of sneaking around their prey and even stealing some of their food from time to time. If a person is sly as a fox, they are extremely clever and cunning in their deception.