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A situation of total disorder. From the BBC satire 'The Thick of It'. Now in common currency in the UK - not sure if overpond usage observed. For instance it has been used in the House of Commons.

A nice one to bring in to play if 'debacle' is a little prim for the mess you observe, and reasonably self-explanatory.

Omnishambles, wreck, trashed, junked, my desk, closets...

To me, Fibber Magee's closet is the measure of a quintessential shambles.
I know that "shambles" comes from the butcher's street in York, England. The street was covered with piles of bones, skulls, and oufel that a complete carnage was always there. And shambles became ubiquitous for a complete mess. I took a snickel-walk (local historical walk) when I visited it in the 1990s, and Shambles Lane was one of the historical stories they told. I personally think the "omni-" part is wholly unnecessary, there is a famous painting n prints of Shambles in York that once seen you'd know why shambles suffices.

E

The etymology of shambles appears to be a tad different. The lane is named for the butcher's tables, not the offal.

So you might be right, it has been 20 years since I heard the story of the namin of The Shambles in York, England. It could be that with all the cow bones, brickerbrack, et al the namer named that part of town The Shambles, because it looked like a shambles.

E

From the King James Bible translation, it appears that shambles simply meant meat market in the early 1600s.

...I personally think the "omni-" part is wholly unnecessary...

I think it is in this case a little more than simple emphasis - it's a situation that has been comprehensively mismanaged. I'm pretty sure that you have some of those over the pond from my reading of current affairs We certainly have our share. You may or may not consider it a useful prefix, but it has stuck quite assuredly seeming to find a useful place here, and always seems capable of inducing a wry smile.