During my thirteen years of trundling between “the world” (what we called the U.S.) and “in-country” (Vietnam), I worked regularly with army and Marine units on the battlefield. I was under cover as one of them and lived with them and wore their uniforms—slept beside them on the ground, ate C-rations sitting in the dirt next to them, shared their latrines. I heard lots of GI slang during those years. Two examples still make me laugh and wince at the same time.
While talking to the Vietnamese, the soldiers and Marines used “number one” to mean “the best” and “number ten” to mean “the worst.” When I asked them where that terminology originated, they told me they got it from the Vietnamese. I asked the Vietnamese where the terms came from. They told me “from the Americans.”
It’s true that in Vietnamese, the word nhât with a rising tone following a word used as a modifier indicates the superlative. Nhât by itself means “one.” But the words meaning “number one” or “the first,” dê nhât, are never used to mean “the best.” They simply designate the first in a series.
The GI slang term “number one” comes from the time of the American occupation of Japan at the end of World War II. The Japanese do indeed use the term “number one” or “the first” to mean “the best.” In Japanese, it’s Ichiban. But the Japanese do not, as far as I know, use “number ten” to mean “the worst.” That, apparently, was a GI invention. American GIs incorporated both terms into their lingo. Senior non-commissioned officers who learned “number one” and “number ten” from their predecessors began using them in Vietnam in the 1960s with the Vietnamese. Their unconscious assumption was that all Asians are alike and must speak the same way.