"coffee," by 1932, likely derived from Java, a noted source of fine coffee, as explained in the glossary of naval terms in Robert P. Erdman, "Reserve Officer's Manual, United States Navy" (Washington, 1932). The guess that it is from the name of U.S. coffee merchant Joseph Martinson (c. 1880-1949) is not chronologically impossible, but it wants evidence and seems to have originated in the company's advertisements (1972).
Earlier in American English (1772) it was the colloquial name of a Portuguese or Brazilian coin worth about $8, shortened from Johannes in this sense (1758), the Modern Latin form of Portuguese João (see John), name of a king of Portugal whose head and Latin inscription appeared on the coin.
pet-form of Joseph (q.v.). Meaning "generic fellow, man" is from 1846. 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