1936 (adj.), Amer.Eng., apparently an abbreviation of Government Issue, applied to anything associated with servicemen. Transferred sense to "soldiers" during World War II (first recorded 1943) is from the jocular notion that the men themselves were manufactured by the government. An earlier G.I. was an abbreviation of Galvanized Iron in G.I. can, a type of metal trash can, the term being picked up by U.S. soldiers in World War I as slang for a similar-looking type of German artillery shells. This use is attested from 1928, but it is highly unlikely that this 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.