If you're devious or tricky, you might be described as sneaky. Tiptoeing up behind your napping dad to scare him would be sneaky.
You might watch your sneaky cat stalking an oblivious squirrel, or act sneaky yourself — slipping quietly into your brother's room to borrow his favorite sweater without asking first. Anytime you make sure not to be seen doing something, you're being sneaky. Sneaky most likely has roots in the Middle English sniken, "to creep or crawl."