at the mercy of

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at the mercy of

1. (idiomatic) In the power of; defenceless/defenseless against.
   • 1897 December (indicated as 1898), Winston Churchill, chapter I, in The Celebrity: An Episode, New York, N.Y.: The Macmillan Company; London: Macmillan & Co., Ltd., →OCLC:

     The stories did not seem to me to touch life. […] They left me with the impression of a well-delivered stereopticon lecture, with characters about as life-like as the shadows on the screen, and whisking on and off, at the mercy of the operator.

   The ball game is scheduled for Saturday, but we're still at the mercy of the weather.