"Gung ho" is a Chinese factory slogan that became a Marine battle cry.
Merriam-Webster traces it to Chinese gōnghé, a shortened form of Zhōngguó Gōngyè Hézuò Shè, the Chinese Industrial Cooperative Society. The movement began in Shanghai in 1937 as a response to the Japanese invasion. Wordorigins.org says "work together" came from the two characters on the cooperatives' triangular trademark.
Marine Lieutenant Colonel Evans Carlson knew the cooperatives from his time in China as a military observer. In 1942 he commanded the new Second Marine Raider Battalion and chose the phrase as its motto. HistoryNet says he taught it to his raiders as a call to work together, though an Associated Press report of August 1942 rendered it "Work and Harmony." Etymonline quotes a 1942 New York Times Magazine piece saying Carlson held what he called "kung-hou" meetings.
The translation is disputed. Merriam-Webster says "work together" is inaccurate, because the two syllables cannot be combined that way in Chinese. The phrase was a clipped name, much as English shortens a long title. Word Histories treats the gloss as the standard translation of the period, repeated in American print from 1941.
Carlson is often credited with bringing the phrase into English, and the record says otherwise. Wordorigins.org says it was already used by English speakers before he adopted it. A Washington Sunday Star article of November 19, 1939, spelled it "Kung Ho," and the "gung-ho" spelling appears in the Evening Star on February 19, 1941. Carlson popularized it and tied it to the military. He did not introduce it.
Fame came with the war. After the Second Raider Battalion raided Makin Atoll in August 1942, press coverage spread the term widely. A 1943 Hollywood film, Gung Ho!, starring Randolph Scott, was loosely based on the raid.
In the process the meaning moved from industry to the military, and then to mood. The phrase later stuck in English as an adjective for extremely enthusiastic or overly zealous. The cooperative's message, working together, dropped out of the word almost completely.