Английский Этимологический словарь - g.i.

- 1936, Amer.Eng., apparently an abbreviation of Government Issue, applied to anything associated with servicemen. Transfered sense to "soldiers" during World War II is from the jocular notion that the men themselves were manufactured by the government. Another theory traces it to Galvanized Iron, and holds that a G.I. can was a metal trash can, which was picked up by U.S. soldiers in World War I as slang for German artillery shells. But this doesn't explain how it came to mean "soldier." I probably get more e-mail about this entry than any other. No two sources I have agree on the etymology, but none backs the widespread notion that it stands for General Infantry.