Green’s Dictionary of Slang

(US) an insane person.

1955		S. Stern Rebel Without a Cause [film] Got a kookaboo inside with a gun [HDAS].
1959		H. Ellison ‘Sally in Our Alley’ in Gentleman Junkie (1961) 123: The weirdest part of all [...] was the cuckaboo with the hood.
1996		M.D. Alder Robotics 🌐 See the various books by Richard Dawkins, in particular The Selfish Gene, The Blind Watchmaker for the contemporary educated answer to the question ‘What is the meaning of Life?’ See any born again cuckaboo for an uneducated answer.
2002		U. Dürmüller ‘Sl. Phonology’ 🌐 Examples of voice alternation in American slang (voiceless-voiced): kookapoo = kookaboo ‘lunatic’.