The term 'KEYSTONE BUMBLER' is a combination of two words, 'KEYSTONE' and 'BUMBLER'. KEYSTONE refers to the central stone in an arch that holds the other stones in place, while BUMBLER is a term used to describe someone who is clumsy or awkward. Together, the term 'KEYSTONE BUMBLER' is used to describe someone who is clumsy or awkward but is also essential to the success of a project. The crossword solution for the term KEYSTONE BUMBLER is KOP, which stands for KEYSTONE of the Project.