The "funny bone" is neither funny (when you whack it on the edge of a piece of furniture), nor a bone (whether you whack it or not). Where'd the name come from, and why is it so painful?
What it is
The funny bone is actually the ulnar nerve, a nerve that runs from the neck all the way to the hand, where it innervates several muscles in the hand and forearm and ends in two branches that innervate the pinkie and half of the ring finger.
Why it hurts
For most of its length, the ulnar nerve is protected, like rest of the body's nerves, by bones, muscles and/or ligaments. As the nerve passes the elbow, though, it runs through a channel called the cubital tunnel, and here it's protected only by skin and fat, making it vulnerable to bumps. When you hit your funny bone, you're actually hitting the nerve against bone and compressing it. The result is an exhilarating cocktail of numbness, tingling and pain that shoots through the areas where the nerve does its work: down the forearm and hand and into the ring and pinky fingers.
Playing the name game
There are two camps in the debate over how the ulnar nerve got be known as the funny bone. One side says that it's an anatomical pun, because the nerve runs along the humerus, which sounds like "humorous." (Get it?) The other side claims that the nerve got its nickname because of the funny (as in odd) feeling you experience after you hit it.
Things could be worse
When you hit your funny bone, it seems like the worst thing in the world, but imagine experiencing chronic irritation there, like someone banging on your funny bone day and night. It sounds like a method of torture you'd dream up in a revenge fantasy, but it's a very real problem called cubital tunnel syndrome.
Cubital tunnel syndrome occurs when the ulnar nerve is obstructed during its trip along the elbow and gets pinched or squeezed, usually from sleeping with the arm folded up. The result is the same as a quick whack to the funny bone: numbness, pain and a tingling sensation, but it lasts a bit longer. Over time, progressive irritation of the nerve causes the numbness to settle in and stay. Muscle weakness in the forearm and hand can also set in, and the pinkie and ring finger can curl up in a position called the "ulnar claw." The condition can usually be helped with elbow splinting and the correction of aggravating postures, hand therapy or, in extreme cases, surgery that provides more space for the nerve and reduces the amount of pressure on it.