Gone to rat shit 

This expression was in common use in the British Royal Navy, certainly in the eighteenth century, but is probably even earlier than this. As these ships were built of wood and powered by sail, emphasis was given to fire power and hull design and as a result crew accommodation and food storage were extremely primitive. This meant that foodstuffs were subject to attack and contamination by the numerous rats living on them. When this happened, the food or other stores, such as sails or ropes that had been destroyed by rats or so contaminated by them as to be totally unusable, would be described as having 'gone to rat shit'.
The expression became more widespread due to its use within the Royal Dockyards and still later within civilian establishments. Its meaning also began to change and expand so that it came to mean anything that had become broken, damaged or useless for any reason not just rat attack. It also began to be applied to people where it meant someone who had changed from being pleasant and personable to unpleasant and disagreeable. It is also applied to someone whose health is failing, affecting their appearance.

Some Americans believe that since Donald Trump’s election everything has gone to rat shit.

Gone to rat shit by AKACroatalin February 24, 2017

Gone to rat shit 

This expression originated with the British Royal Navy from the time of the wooden sailing ships. Storage of food aboard these vessels was quite primitive and foodstuffs were subject to attack by the numerous rats living on them. Frequently food, or other stores, would be destroyed by rats or so contaminated by them as to be totally unusable, the crew would then say that whatever had been destroyed had 'gone to rat shit'.

The expression became common in the Royal Dockyards where its use expanded to mean anything that had become broken or unusable whatever the cause. The use of the expression further expanded to mean someone who had changed from being a pleasant person to an unpleasant, grouchy bastard, or someone whose health was beginning to fail.

Malcolm's made another major fuck-up with the website, it's all gone to rat shit.
Yeah, when Sally told him about it, he screamed at her like it was her fault! He never was very nice, but now he's really gone to rat shit.
Malcolm's gone to see his doctor again, he really has gone to rat shit.

Gone to rat shit by Croatalin December 20, 2013