What are you ordering if you ask for "a cup of Joe"?

Coffee is a beloved drink of millions of people around the world. It is so loved that it has spawned all sorts of endearing nicknames. Two of the most common are "java" and "cup of joe." Historians believe that the "cup of joe" didn't first enter the English language until about 1930. That common nickname has been around for a long time, but its origins are still mysterious. Several theories have been put forth, but none of them can claim to be the definitive explanation.

Source: Wonderopolis