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"mentally unbalanced person," by 1900, probably from crack + pot (n.1) in a slang sense of "head." Cf. crack-brain "crazy fellow" (late 16c.). Earlier it was used in a slang sense "a small-time big-shot" (1883), and by medical doctors in reference to a "metallic chinking sometimes heard when percussion is made over a cavity which communicates with a bronchus."
: my colleague's crackpot notionsnoun
A crazy idiot; an addled fool; eccentric: He's a crackpot about flying saucers (1883+)