To cancel, reverse, or overrule a previous order or command that someone has given.
From French 'contremander,' combining 'contre-' (counter, against) with 'mander' (to command, from Latin 'mandare'). Used since medieval times when military commanders needed to reverse battle orders.
Countermand comes from military history—when a general shouted an order but then realized it was wrong, messengers would countermand it across the battlefield. It's the origin of military communication protocols!
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