Green’s Dictionary of Slang

crack n.2

[one whose brain has a fig. ‘crack’]

a fool.

[UK]Dekker Lanthorne and Candle-Light Ch. 2: A Foyst nor a Nip shall not walke into a Fayre or a Play-house, but euery cracke will cry looke to your Purses.
[UK]The Wandering Jew 40: Who is this Ioculo? A melancholy Hee-Cat (sir) said the cracke.
Apollo’s Feast 24: Will, as arch a crack as any in this Town.
[UK]J. Addison quoted in Annandale n.p.: I cannot get the Parliament to listen to me, who look upon me as a crack [F&H].
[US]Da Bomb [Internet] 7: Crack: Nutcase; a person who lacks good judgment.