Wordorigins.org

Most people nowadays know that a G.I. is an American soldier and that the term is popularly associated with the Second World War, but few know what the abbreviation G.I. originally stood for or that the term predates WWII by some decades.

G.I. was originally a semi-official U.S. Army abbreviation for galvanized iron, used in inventories and supply records. It dates to at least 1907 and is commonly found in records from the First World War. From Col. Frank P. Lahm’s Diary of 1917:

[Lympe, England] is a large depot where machines are delivered for forwarding to France. 12 large hangers [sic], brick, G.I., about 75 ft wide by 150 ft long.