Green’s Dictionary of Slang

a drunkard.

1909		J. Ware Passing Eng. of the Victorian Era 82/2: Cod (Trade. Tailors’). A drunkard. The word is suggested by the fallen cheeks and lips’ corners which are some of the facial evidences of a drunkard, and which certainly suggest the countenance of a cod, which fish, furthermore from its size, is typical of huge drinking.
1966		P. Boyle At Night All Cats Are Grey 75: Don’t encourage the old cod.