All get-out is a mass noun which means to an extreme. It is used in comparison to another item, usually with the word as. Get-out is hyphenated in one dictionary entry, however, most users do not include the hyphen and simply make the phrase in three words instead of two.
The phrase originates in the late nineteenth century without the word all (e.g., as getout). One of the first instances of its current form in print is the American novel Huckleberry Finn. The author, Mark Twain, was famous for his descriptions of Southern life and his use of common vernacular instead of the proper English of the time.
The phrase’s similarity to get out of here should be recognized; however, there is no evidence one affected the other, though it is possible.