'OK' is the most spoken and most written word in the entire world. It's such a fundamental part of modern communication that it's hard to imagine the world without it. But in spite of its ubiquity and compact versatility, 'OK' is under two hundred years old. Today's episode tells the story of the word's origins in 19th century America. If the leading theory is correct, then OK might just be the most successful inside joke of all time.