If you're unbelted, you're not strapped securely in your seat. Don't even pull the car out of the driveway if any of your passengers are unbelted!
The adjective unbelted means "without a belt" or "with an unbuckled belt." So if you haven't clicked your seatbelt yet, you're unbelted, and if your pants are falling down, it may be because you're unbelted. This word is formed by adding the prefix un-, "not," to belted, "secured by a strip of material around the waist."