To dig or burrow in opposition to an enemy's digging, especially in military mining operations.
From counter- + ermine, which is not from the animal but from Old French 'ermin,' related to mining terms. Medieval military term for defensive mining against enemy sappers.
During medieval sieges, soldiers literally had underground warfare—if enemies were tunneling toward your walls, you'd counterermine by tunneling to intercept them, creating underground battles you'd never see.
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