Plural of billy; a container or can used for cooking, especially by campers or soldiers, or a billy goat.
From 'billy' (short for William), which became a generic term for a drinking vessel in Australian English. The cooking container sense comes from 19th-century military and camping usage where personified items were given common names.
Australian soldiers called their cooking cans 'billies,' and this slang became so standard that it's now the official term in Australian English—a perfect example of how everyday camp culture can reshape a language's vocabulary.
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