Astern means at the rear of a ship, boat, or plane. If your seat on an airplane is astern, you'll have to walk all the way to the very back to find it.
While you can use astern to mean "at the back" or "behind," it's most common to save this adjective for when you're on a sailboat or describing the tail section of an airplane. The word was originally nautical, meaning "toward the stern of the boat," or "at the back of the ship." One theory about the origin of astern and stern says their root is the Old Norse stjorn, "a steering."