Any idea where this expression, “to get your ass in a sling”, comes from?

In medieval times, attackers besieging a castle would load dead livestock from the surrounding farmland into trebuchets and hurl them into the castle.  ;)

Or maybe it’s just a fanciful combination of the images of an injured arm supported in a sling and of the buttocks as the target of punishment and life’s other difficulties.  The OED cites it from 1960 in Flexner and Berg’s Dictionary of American Slang.

The American Heritage Dictionary of Idioms offers an unsubstantiated guess, with a slightly earlier date:

“Probably originating in the American South, this idiom may refer to so vigorous a kick in the buttocks (for which ass is a rude synonym) that the injured person requires a sling of the kind used to support a broken arm. [Vulgar slang; c, 1930]”. 

That still leaves me wondering about the origin. 

Dead livestock?  I thought they were all serving in high political office.

“Probably originating in the American South, this idiom may refer to so vigorous a kick in the buttocks (for which ass is a rude synonym) that the injured person requires a sling of the kind used to support a broken arm. [Vulgar slang; c, 1930]”.

That still leaves me wondering about the origin.

Huh? The quote gives the time and place of probable origin and the probable explanation; what kind of “origin” does it leave you wanting?  “The phrase was first uttered by Hamhock Henderson of Sowsear, Alabama, on July 20, 1928, as part of a threat directed at his neighbor Fred A. Smith”?  I don’t think you’re going to do much better than your quote.

Oddly, it doesn’t seem to be in HDAS, unless I’ve missed it in scanning the pages of ass idioms.

It would have been under “sling” rather than “ass” in HDAS, since what is in the sling is variable: ass, butt, cock, balls, or even an entire person: “Don’t go off half-cocked and get yourself in sling.” Uses from the 1880s have things like eyes and noses in slings, jocularly meaning “injured,” since neither body part can really be in a sling. The citation evidence would have shown a pretty consistent trail from jocularly meaning injured to figuratively meaning injured to meaning “in trouble.”

since what is in the sling is variable

Thanks for that explanation. I’ve always wondered why HDAS alphabetized by words other than the first in some cases. I’ve been frustrated by this in the past. It will make finding things faster in the future.

I add my thanks.  Obvious once you’re told!

Thanks to all, especially to Mr. Barrett.

since what is in the sling is variable: ass, butt, cock, balls, or even an entire person:

I’ll bet that prepositional phrase doesn’t show up on the radio program.  Thanks Grant.  That was indeed helpful.