Reverse one's upward course, descend. For example, When she saw the wasps' nest on the roof, she hastily backed down the ladder. This literal usage usually refers to something one has climbed, such as a ladder or mountain. [Mid-1800s]
Also, back off. Retreat or yield. For example, As the watchdog began to snarl the letter carrier backed off, or You have a good point; now don't back down when you present it to the board. [First half of 1900s] Also see back away, def. 2.