Green’s Dictionary of Slang

moist n.

(US) an alcoholic drink, thus moist drink.

1899		‘Pot’ & ‘Swears’ Scarlet City 544: Charlie, come and mash me for a bit, my Johnny’s gone to get a toothful of moist.
1902		C.L. Cullen More Ex-Tank Tales 40: My friend was still riding and purchasing Gallic moists.
1908		Hickman Courier (KY) 12 Nov. 5/6: The manufacturer is making an effort to introduce it in territory where the ordinary moist drink is prohibited.

In compounds

moistmill (n.)

(US) a bar.

1902

C.L. Cullen More Ex-Tank Tales 43: The darky bootblack whose plant was outside the moistmill.

moist ’un (n.)

(US) a drunkard.

1933

‘William Juniper’ True Drunkard’s Delight 233: A term for a drunkard [...] moist’un.