A few minutes ago I checked the dictionary to see if the word gung-ho is hyphenated. (Yes, it is.) Reading a bit further, I was fascinated to find the following etymology:
introduced as a training slogan in 1942 by U.S. Marine officer Evans F. Carlson (1896–1947) < Chinese gōng hé, the abbreviated name of the Chinese Industrial Cooperative Society, taken by a literal translation as “work together”
Wikipedia adds a bit more detail:
The term was picked up by United States Marine Corps Major Evans Carlson from his New Zealand friend, Rewi Alley, one of the founders of the Chinese Industrial Cooperatives. Carlson explained in a 1943 interview: "I was trying to build up the same sort of working spirit I had seen in China where all the soldiers dedicated themselves to one idea and worked together to put that idea over. I told the boys about it again and again. I told them of the motto of the Chinese Cooperatives, Gung Ho. It means Work Together-Work in Harmony...."
Later Carlson used gung ho during his (unconventional) command of the 2nd Marine Raider Battalion. From there, it spread throughout the U.S. Marine Corps (hence the association between the two), where it was used as an expression of spirit and into American society as a whole when the phrase became the title of a 1943 war film, Gung Ho!, about the 2nd Raider Battalion's raid on Makin Island in 1942.
Maybe this origin story is common knowledge. I don't know—though I've never seen it on Jeopardy! or in a crossword puzzle. In any case, it was new to me, so maybe it's new to you too. It's certainly interesting that our modern-day use of the word has approximately nothing to do with working together, which I suppose is just a linguistic casualty of war.