Googling for this has yielded mostly conjecture, common and plausible versions of which include: "get-out" is a euphemism for hell (as in "get outta here" in response to saying an offensive word); and "get-out" refers to an implausible escape.
Etymonline
lists get-out in the sense of a high degree of something, but offers no background information.
The expression seems to have originated as simply "as get-out" and then migrated to the more common modern version. According to the
American Heritage Dictionary of Idioms, the earliest attested usage is from Joseph C. Neal's
Charcoal Sketches (although the dictionary lists the title as
Character Sketches). This matches the date given at etymonline of 1838.
Any resident language experts have any further info on this?
posted by languagehat at 5:25 PM on April 19, 2011