A person, group, or organization so powerful (either by size or by influence) that it does not need to heed the rules or threats of others. It refers to the riddle "Where does an 800-pound gorilla sit?" (Answer: "Wherever it likes.") Primarily heard in US. Don't mess with that guy; he's like an 800-pound gorilla! These big corporations act like 800-pound gorillas, making up the rules to suit themselves.