The U.S. expression "Gung Ho!" was introduced as a battle cry by Marine Corps Col. Evans Carlson in 1942. Carlson, leader of the USMC First Raider Battalion, had been an observer with Chinese Communist forces just before America entered the war. Carlson erroneously believed that "Gung Ho!" was Chinese for "Work Together!" and this is the meaning it was given in Carlson's unit.
By the end of the war it was being used as an adjective, and had already begun to acquire an ironic tinge. It is now used throughout the American military both seriously and ironically, depending on circumstances.