The theories have names that seem almost to be begging ridicule—the Bow-Wow theory, the Ding-Dong theory, the Pooh-Pooh theory, the Yo-He-Ho theory—and they are generally based in one way or another on the supposition that languages come ultimately from spontaneous utterances of alarm, joy, pain, and so on, or that they are somehow imitative (onomatopoeic) of sounds in the real world.
If onomatopoeia explains the origins of certain words even today, perhaps this is how the oldest ones began?
