Also used by U.S. soldiers as a verb. To G.I. something means roughly to clean it until it is spotless, then clean it some more, and then make it shiny. Most often used in reference to floors and other parts of living quarters. It can also mean to hastily rig a solution to a problem, in roughly the same usage as the computer programming term "hack".

A G.I. is a US Army soldier.

G.I. is an abbreviation for "General Infantry", and usually refers to the period of 1915-45, during World Wars I and II. Most G.I.'s entered the service as privates.

Galvanized Iron.

Abbreviated and stamped on certain military items, especially during World War II. Frequently misinterpreted as General Issue or Government Issue, thus the original irony when referring to a soldier as "a G.I." -- a standardized, mass-produced, government-approved human being.

Other explanations for the term are examples of backronyms.

In medicine it stands for "Gastro-Intestinal", or "Gastrointestinal", (i.e. the digestive system).