crackpot (n.)

"mentally unbalanced person," 1898, probably from crack (v.) + pot (n.1) in a slang sense of "head." Compare 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."