Green’s Dictionary of Slang

plimsoll n.

SE in slang uses

In phrases

over the plimsoll (adj.) [the SE plimsoll line, marking the limit of loading a ship]

(N.Z.) drunk.

1921

E.I. Lord ‘Stunology’ in Ballads of Bung (1976) 11: ‘Over the plimsoll’, ‘rocking’ or that ‘you’re on the roll’ – All these expressions clearly show you love the flowing bowl [DNZE].