:in a position or situation in which you can be harmed by (someone or something you cannot control)任…处置;对…无能为力;任由…摆布
With no way to control the ship, we were at the mercy of the sea.由于没有办法控制我们的船,我们只好任由大海摆布了。
The people were at the mercy of the advancing army. = The army advanced, and the people were at their mercy. [=the people could do nothing to defend themselves from the army]对于攻上来的敌军,人们无能为力。
Our plans were at the mercy of the weather.我们的计划能否实施只能看天气了。