"coffee," by 1941, perhaps late 1930s, of unknown origin. Meaning "generic fellow, man" is from 1846, from the pet-form of Joseph, q.v.. Used in a wide range of invented names meaning "typical male example of," for example Joe college "typical college man" (1932); Joe Blow "average fellow" is U.S. military slang, first recorded 1941. "Dictionary of American Slang" lists, among other examples, Joe Average, Beige, Lunch Bucket, Public, Sad, Schmoe, Six-pack, Yale, Zilch