funny bone 
noun informal
The part of the elbow over which passes the ulnar nerve, which may cause numbness and pain along the forearm and hand if knocked: she banged her funny bone on the doorpost...
- I'm here to say, would you all pick up your elbow and whack your funny bone and feel that pain that stops you in your tracks from doing what you're doing?
- He grabbed it and rubbed his funny bone vigorously until the pain ebbed.
- I tried to give him the thumbs up but banged my funny bone on the pole and had to shriek in pain.