This technical military term denotes a protective covering used by attackers storming a city—what Romans later called a testudo or tortoise formation. Nahum uses it in his vivid oracle about Nineveh's fall: 'the covering is prepared.' The structure shielded soldiers from defensive projectiles as they approached walls or gates. The term literally means 'that which covers or screens,' showing how warfare technology included sophisticated defensive equipment. The prophet's point is that even with proper military preparations, Assyria's capital cannot escape divine judgment.