The “funny bone” is that part of the elbow over which the ulnar nerve passes. A knock on the funny bone may cause numbness and pain along the forearm and hand.
“Funny bone” is also an idiom meaning a person’s sense of humor. When something tickles your funny bone it makes you laugh or amuses you.
The phrase has been in use since the early 1800s.
The idea that “funny bone” was derived from the name of the upper arm bone was, instead, more likely a joke enjoyed by anatomy students after the idiom was already common. As explained by James Mitchell in Significant Etymology: Or, Roots, Stems, and Branches of the English Language (1908):
“The funny bone, or, as Americans more frequently term it, the crazy-bone, is the term popularly applied to what anatomists call the inner condyle of the L. humerus…a blow on which jars the ulnar nerve and produces a funny tingling sensation. A good dissecting-room joke for first-year’s students is, Why is the funny-bone so-called? ‘Because it borders on the humerus.’ This jest is seriously taken by a recent etymologist who explains the word funny-bone as being a pun on the word ‘humerus.'”
Although this may be true, it seems unlikely that an idiom based on a knowledge of human anatomy could have arisen in the early 1800s. The idiom probably derived from the “funny” feeling one experiences from hitting the area described in definition one above. From there, a transference to the more common sense of the word funny, meaning amusing, would easily follow.
The related idiom “tickle one’s funny bone,” used at least since the early 1900s, derives from combining the connotation of humor with the fact that one laughs when tickled.
Reba McEntire, actress and singer, said, “To succeed in life, you need three things: a wishbone, a backbone and a funny bone.”
I couldn’t agree more.