E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A pun on the word humerus. It is the inner condyle of the humerus; or, to speak untechnically, the knob, or enlarged end of the bone terminating where the ulnar nerve is exposed at the elbow; the crazy bone. A knock on this bone at the elbow produces a painful sensation.