What does "bugaboo" mean?

Bugaboo: Something frightful, as a specter; anything imaginary that causes needless fright; something used to excite needless fear; also, something really dangerous, used to frighten children, etc. "Bugaboos to fright ye." Lloyd. But, to the world no bugbear is so great As want of figure and a small estate. Pope. The bugaboo of the liberals is the church pray. S. B. Griffin. The great bugaboo of the birds is the owl. J. Burroughs.