Green’s Dictionary of Slang

clay n.

[mid-18C–19C] a clay pipe.

SE in slang uses

In compounds

clay-assed (adj.)

[1960s–70s] (US) stupid, peasant-like.

clay-brained (adj.) (also clay-pated)

[late 16C; 1980s+] stupid; thus clay-brains, a fool.

clay-eater (n.) [the literal eating of clay by such people in order to supplement their otherwise meagre diet]

[mid-19C+] (US) a poor white, esp. a native of North or South Carolina or Georgia; thus clay-eating adj.

clay-puncher (n.)

[late 19C] (Aus.) a miner.

clay-punching (n.)

[1900s] (Aus.) working as a miner.

In phrases

moisten the clay (v.) (also damp the clay, soak one’s..., wet the...) [SE clay, the human flesh]

[early 18C–1950s] to take a drink, to quench one’s thirst.