"Rat arsed" is a British slang term that means "very drunk" or "intoxicated".
The phrase "rat arsed" is a play on words, as "rat" is used to describe something dirty or undesirable, and "arsed" is a slang term for the buttocks. The phrase is often used humorously, but can also be used in a negative context to describe someone who is out of control due to excessive alcohol consumption. It is considered informal language and may not be appropriate for formal situations.