Green’s Dictionary of Slang

heavy wet n.

also heavy, heavy whet
[‘Heavy wet, malt liquor, because the more a man drinks of it, the heavier and more stupid he becomes’ (Hotten, 1867)]

1. [early 19C] a heavy drinking bout; also attrib.

2. [early 19C+] malt liquor; also a mixture of porter and beer.

3. [early 19C-1940s] a downpour, a rainstorm.