Definition of "crackbrained"

In order to ſet ſo hopeful a Genius right in this matter, vve have ſent you an anſvver to the ill-grounded Sophiſms of thoſe crack brain'd fellovvs, and likevviſe an eaſy mechanical explication of Perception or Thinking.

1713–1714 (date written), Martinus Scriblerus [pseudonym; Alexander Pope et al.], “Memoirs of the Extraordinary Life, Works, and Discoveries of Martinus Scriblerus”, in The Works of Alexander Pope Esq. […], volume VI, London: […] J[ohn] and P[aul] Knapton […], published 1751, chapter XII (How Martinus Endeavoured to Find Out the Seat of the Soul, and of His Correspondence with the Free-thinkers), page 151

Could it be, after all, that the whole story was true, and the writing on the sherd was not a forgery, or the invention of some crack-brained, long-forgotten individual?

1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887

He shook his head; the milk pail was still at his feet. «You always been a crackbrained sonofabitch anyhow.»

1957, Jack Kerouac, On the Road, Viking Press

Ralph is a downtrodden, debt-ridden mailman who lives on a diet of Jeopardy, Wheel of Fortune, and The People's Court; dreams of sudden quiz-show success; and (like his namesake, Kramden) keeps hatching crackbrained get-rich schemes (his latest is a battery-operated jump rope).

1988 February 5, Lawrence Bommer, “The Fan Club”, in Chicago Reader‎