When you snoop, you poke around in someone else's business. A teenager might snoop in her sister's room, looking for her diary.
You snoop when you rustle through someone's garbage looking for love letters, and if you spy on your neighbor through the curtains, you also snoop. In fact, if you spend enough time doing this, you'll be called a snoop — a sneaky busybody. The earliest meaning of snoop, "to go around in a prying manner," comes from the Dutch word snoepen, "to pry."