countermine n (military) a tunnel dug to defeat similar activities by the enemy v destroy property or hinder normal operations v destroy enemy mines with one's own mines We countermined the banks of the river Usage(s) During the final siege of the city, the attackers burrowed beneath the walls in order to breach the Roman defenses; the Romans heard this and started digging a countermine to fend . Synonym(s) sabotage undermine counteract subvert weaken