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.