[mid-18C–19C] a clay pipe.
SE in slang uses
[1960s–70s] (US) stupid, peasant-like.
[late 16C; 1980s+] stupid; thus clay-brains, a fool.
[mid-19C+] (US) a poor white, esp. a native of North or South Carolina or Georgia; thus clay-eating adj.
[late 19C] (Aus.) a miner.
[1900s] (Aus.) working as a miner.
[early 18C–1950s] to take a drink, to quench one’s thirst.