at the mercy of
 at the mercy of ( or   at someone's or something's mercy)
  : in a position or situation in which you can be harmed by (someone or something you cannot control)
   With no way to control the ship, we were at the mercy of the sea.
   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]
   Our plans were at the mercy of the weather.
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 Main Entry: mercy