The Anonymous Coward is the original default username for editing this wiki, assigned by the software. So if somebody edited the wiki without signing in, then their edits would be attributed to the Anonymous Coward. We now write simply Anonymous. A few jokers have changed ‘AnonymousCoward’ to something different but equally anonymous … which is OK too.
However, people who really want to be anonymous should know that every editor's IP address is (usually) listed in the entry’s history (found by clicking history at the bottom of any page).