Slyness is a quality of being sneaky and smart. It takes slyness to pull off a really good practical joke.
The noun slyness is useful when you're talking about a person's cunning or craftiness. You best friend's slyness will come in handy when you're sneaking into your sister's room to borrow her clothes. Slyness comes from sly, which stems from the Old Norse sloegr, "cunning or crafty."