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.
It’s gripping as all get-out, and seeing Keri Russell in a dramatic adult role is just cool. [Rotten Tomatoes]
Angie and Trevor Spencer, the hosts of Marathon Training Academy, are cute as all get-out. [Huffington Post]
The show was in a kind of sustained conversation with both serious ethical questions and recent headlines—Benghazi, Pakistan, drone strikes—and yet it was tightly plotted, high-octane, and entertaining as all get out. [Slate]
“He has no idea what he’s doing, folks, but he is as gifted as all get out,” one TV analyst said. [The Independent]
The first person we picked was Olly, and obviously he’s as English as all get out. But you just go with it. [The Guardian]