When someone shows sneakiness, they are being sly or trying to do something without being seen.
Sneakiness is usually not good because it means someone is being tricky or dishonest.
Examples:
His sneakiness was revealed when he tried to take the cookies without asking.
I don't like her sneakiness when she tries to peek at my answers.
The cat's sneakiness helped it catch the mouse.
The sneakiness of the fox was no match for the clever rabbit.
Her sneakiness was so great that she could steal anything without anyone noticing.
The sneakiness of the spy was legendary in the secret agent world.