A secondary or reserve tank used in mechanical or industrial systems to provide additional fluid storage capacity.
Compound of 'co-' (together) plus 'pin' (possibly referring to a coupling mechanism) plus 'tank,' though the exact origin of 'pin' in this context is uncertain and may be dialectal.
This appears to be a specialized industrial term with limited modern usage, possibly representing older engineering terminology from the steam age when backup systems were essential for preventing catastrophic failures.
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