The popular phrase “cup of joe“, referring to a cup of coffee, was made popular during WWII. Enlisted men (General Infantry) in the U.S Army were fondly referred to by the slang phrase G.I. Joe, made popular by a comic strip distributed by King Features Syndicate, which first appeared in newspapers in 1942. Coffee was an immensely popular beverage consumed by enlisted men during WWII, hence the establishment of the popular phrase “cup of joe” from G.I. Joe.
Personal note: Thanks to all the G.I. Joes