Q: What is the origin of “from the git-go,” which I believe is a military expression that means from the very beginning?
A: “From the git-go” is a variation on “from the get-go,” which is a colloquial version of “from the word go,” according to the Random House Historical Dictionary of American Slang.
Another source, Cassell’s Dictionary of Slang, suggests “get go” is related to “get going.”
Whatever the phrase’s derivation, it means, as you mention, from the beginning. I don’t see any indication, however, that the expression in any of its forms is limited to the military.
The first citation in Random House comes from Toni Cade Bambara’s short story “The Hammer Man” (1966): “I knew Dick and Jane was full of crap from the get-go.”
And here’s a citation using the spelling “git-go,” from Richard Woodley’s novel Dealer (1971): “It was his bust from the git-go.”
Both Cassell’s and Random House say the expression originated in Black English.