I’ve been wondering how the word joe came to be used in a slang sense for coffee. I consulted Evan Morris’ book The Word Detective to see what his theories are.
In fact, no one seems to know for certain. It may be that joe is somehow associated with the island of Java, since java is another synonym for coffee. In the 19th century, the Indonesian island of Java was a major source of the world’s coffee.
Joe is often used to refer to the average man, the common man (his female equivalent is Jill), and has been especially associated with the military (we all know GI Joe, slang for the common soldier long before he hit the toy store shelves). Because coffee is said to fuel the military, an association between common soldiers and their drink of choice is fixed.