If you're unaccustomed to something, you're not used to it. When you're unaccustomed to riding a horse, it might take a while to get the hang of staying in the saddle.
An unaccustomed knock on the door might startle you and make your dog bark in alarm, and an unaccustomed bonus in your paycheck will come as a happy surprise. You might feel awkward having tea with a princess in a palace if you're unaccustomed to fine dining and dignified table manners. The prefix un here means "not," and it's combined with accustom, with its ultimate Latin root word consuetudinem, "habit, usage, practice, or custom."