If someone is sneakier, they are more likely to do things without being seen or caught.
Being sneakier is usually not a good thing. It can make people feel like they can't trust the person.
Examples:
He is sneakier than his brother.
The cat became sneakier when it wanted more food.
The sneakier fox got the most food.
The sneakier the cat, the more food it gets.
She is the sneakiest person in the whole world!
The sneakier he gets, the more trouble he finds.