"tis only pure cat"

Cat Melodeon, when things are real bad

Cat - shorthand for catastrophic

Drogheda too.

Is it not just a misunderstanding and they are trying to say "cack"?

I doubt if it's an abbreviation and acronym in this case.

Here's an elaboration of what Tabby Aspreme said above. (And a Tabby should know about cats. 😉 )

Cat

The small animal, of course, but in slang it means something that looks, sounds or simply is absolutely awful. It’s short for cat malojan, which is probably short for the phrase, cat on a melodeon. You can imagine that a cat standing on a melodeon is likely to make an awful sound. That is probably where the idea came from. Now it’s used to describe anything people don’t like and think is terrible.

(And before anyone says it, I know the cat in the picture isn't on a melodeon. 😀 )