I've selected "zephyr" as a hard difficulty word. It's 6 letters long and contains the uncommon letter 'z' plus an unusual letter pattern. It means "a gentle breeze" but isn't used in everyday conversation. The word has no repeated letters, which adds to the challenge while still being a legitimate English word. It follows all the rules: it's between 4-10 letters, uses only English letters, is a real word (not a proper noun or acronym), and has no spaces or special characters.