Come, consider the wonders of Yahweh, the astounding deeds he has done on the earth; he puts an end to wars over the whole wide world, he breaks the bow, he snaps the spear, shields he burns in the fire. ‘Be still and know that I am God, supreme over nations, supreme over the world.’ Yahweh Sabaoth is with us, our citadel, the God of Jacob (Ps. 46:8-11)
The little phrase ‘be still’ is from a root that means “to sink, relax, sink down, let drop, be disheartened; to sink down; to sink, drop; to sink, relax, abate; to relax, withdraw.”
It denotes a slackening or a relaxing, a settling. It doesn’t connote tranquility or repose, though. Those ideas are found in other roots.
Here the verb seems to suggest that the mighty acts of God require a sort of settling into the knowledge that God reigns and no enemy of his stands a chance. God is Lord, and you are not.