Green’s Dictionary of Slang

1. (also desert mouth) the dry mouth that comes with a hangover; also attrib.

1907		S.E. White Arizona Nights 170: We were both old hands at the business, had each in our time suffered the ‘cotton-mouth’ thirst, and the memory of it outweighed any desire for treasure.
1983		Eble Campus Sl. Mar. 2: cotton mouth – an extremely dry mouth, usually experienced after a long night of drinking. Also [...] desert mouth.

2. (US Und.) a dry mouth caused by fear; thus cotton-mouthed adj.

1944		‘F. Bonnamy’ A Rope of Sand (1947) 209: The very idea makes me cotton-mouthed.
1971		T. Thackrey Thief 371: Cold balls for a minute. Puckered asshole. Cotton mouth. Cops?
1978		S. King Stand (1990) 1185: Trembling, cotton-mouthed, nearly groaning with terror.

3. (drugs) a mouth that has become dry through smoking marijuana; also attrib.

1968		‘Sl. of Watts’ in Current Sl. III:2.
2002		‘Organism’ ‘Sound and Fury’ on Lifeline 14 Feb. 🌐 Pot smokers are funding terra! When they go out to buy ice cream to relieve the cottonmouth munchies, they’ll often purchase...BEN AND JERRY’S!