The quip gets pinned on Groucho Marx and Spike Milligan, but Quote Investigator’s earliest match is an 1830 book on insanity by John Conolly. There “the learned and benevolent Dr. Parr” says of village eccentrics that they were “certainly cracked: but that the crack let in light.” QI doesn’t know Dr. Parr’s first name. A Scottish professor was still passing the line along in 1896, tapping his forehead as he said it.