The story
Further development of pissed as a rat, from the British slang pissed (“drunk”), and drunk as a rat, the animal serving as a generic intensifier. Compare similar ratfaced, pissed as a newt, drunk as a skunk, etc.
First known use, by meaning
- 1980s Synonym of shitfaced, extremely drunk.
Etymology from Wiktionary, CC BY-SA 4.0.