Sly means clever at getting what you want in a sneaky, secretive way. A sly person might quietly take the last cookie when no one's looking, or find a roundabout way to avoid doing chores without actually refusing. There's always something a bit crafty and underhanded about being sly.

In stories, foxes are famous for being sly. They use tricks and cunning rather than strength or speed. When Brer Rabbit outsmarts Brer Fox in the old Uncle Remus tales, he's being sly, using his wits to escape danger. A sly grin or sly smile suggests someone knows something others don't, or has a secret plan.

The word carries a hint of disapproval, though not always. Being sly isn't quite as bad as being dishonest or cruel, but it's not admirable either. If your friend makes a sly comment about your new haircut, they're saying something critical in an indirect way, hoping you might not catch the insult. A sly dog is an old-fashioned phrase for someone who's craftier than they first appear.

Sometimes people use “sly” more playfully, like when a comedian delivers jokes with sly humor, making smart observations that take a moment to sink in.