Origin of the meaning of joe
Solution 1:
Michael Quinion at World Wide Words claims that the use of joe for coffee first appeared in print in 1930. He agrees it's of unknown origin, but outlines two of the more prevalent theories before concluding it's most likely a military-slang modification of other j-words for coffee:
It is significant that an early example appears in 1931 in the Reserve Officer’s Manual by a man named Erdman: “Jamoke, Java, Joe. Coffee. Derived from the words Java and Mocha, where originally the best coffee came from”.
The earliest print reference I could find was from Eugene O'Brien's 1937 naval novel He Swung and He Missed:
Solution 2:
My Webster's lists joe as
- coffee. [ORIGIN: 1940s: of unknown origin.]
- an ordinary man : the average joe. [ORIGIN: mid 19th cent.: nickname for the given name Joseph; compare with Joe Blow .]
Etymonline concurs.
Solution 3:
Seemingly NOT from the Secretary of Navy: http://www.snopes.com/language/eponyms/cupofjoe.asp
My personal guess is java -> joe
http://www.etymonline.com/index.php?term=joe "coffee," 1941, of unknown origin. Meaning "generic fellow, man" is from 1846, from the pet-form of Joseph (q.v.). Joe college "typical college man" is from 1932. Joe Blow "average fellow" is U.S. military slang, first recorded 1941.