Green’s Dictionary of Slang

drunk as a rat adj.

very drunk.

1542		A. Boorde Introduction of Knowledge (1870) 147: Although I wyll be dronken other whyles as a rat?
1553		T. Wilson Rhet. (1580) 128: As dronke as a ratte [F&H].
1583		P. Stubbes Anatomie of Abuses 67: They satte thei swillyng, and carousyng one to an other, till tei were bothe dronke as Rattes.
c.1615		‘The Famous Rat-Catcher’ in Rollins Pepysian Garland (1922) 63: He was so braue a bowzer, that it was doubtful whether He taught the Rats or the Rats taught him to be druncke as Rats togeather.
1617		R. Brathwait Law of Drinking n.p.: When I’m drunk as any rattin, / Then I rap out nought but Latin.
1630		J. Taylor ‘Brood of Cormorants’ in Works (1869) III 5: For though he be as drunke as any Rat, / He hath but catcht a Foxe, or whipt the Cat.
1682		A. Radcliffe ‘The Ramble’ Poems 108: I’ll make him drunk as any Rat.
1691		‘The Tinker of Turvey’ in Ebsworth Merry Drollery Compleat (1875) 28: Drunk as a Rat, you’d hardly wot / That drinking so he could trudge it.
1933		‘William Juniper’ True Drunkard’s Delight 227: He may also, comparatively, be as drunk as [...] a rat.