A small mine shaft or pit dug to explore underground deposits of tin or other minerals.
From Cornish 'costin' or similar Cornish mining terminology, reflecting the language of miners in Cornwall where tin mining was crucial. The word entered English from regional dialect during the mining industry's height in Southwest England.
This is a ghost word from Cornwall's mining heritage—it's so specific to Cornish tin mining that most modern dictionaries barely recognize it, yet it represents a whole lost world of how miners actually talked about their work.
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