A slang or term used mainly in the United States to defined a cup of coffee. The phrase can be traced back to Josephus Daniels, a U.S. Naval Secretary who enforced a policy that banned alcohol aboard Navy vessels. Alcohol consumption was prohibited; the only strong drink allowed was coffee. From then on, coffee became the substitute for alcohol. It is said, that sailors come up with the term out of spite and anger against Josephus Daniels’ general order and started to call coffee a Cup of Joe.
There is another explanation alongside the military tale. According to English language researches, Joe comes from a shortening of Jamoke, which itself is a combination between Java and Mocha. So, where does the nickname Cup of Joe comes from? Most likely, from the shortening of Jamoke, yet we like the Navy story better.