at the mercy of somebody/something

at the mercy of someone

 and at someone's mercy
Fig. under the control of someone; without defense against someone. We were left at the mercy of the arresting officer. Mrs. Franklin wanted Mr. Franklin at her mercy.
See also: mercy, of
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

at the mercy of

1. Also, at someone's mercy. Subject to the power of, helpless against, as in The captured rebels were at the mercy of the army commander. [Late 1500s]
2. Without any protection against, as in On top of Mount Washington we were at the mercy of the elements. [Late 1600s]
See also: mercy, of
The American Heritage® Dictionary of Idioms by Christine Ammer.

(be) at the mercy of somebody/something

not be able to stop somebody/something harming you because they have power or control over you: Small businesses are completely at the mercy of the banks these days. The little ship was at the mercy of the storm.
See also: mercy, of, somebody, something
Farlex Partner Idioms Dictionary

at the mercy of

Without any protection against; helpless before: drifting in an open boat, at the mercy of the elements.
See also: mercy, of
American Heritage® Dictionary of the English Language, Fifth Edition.
See also:
  • get (someone) by the balls
  • go for the throat
  • place (oneself) at (someone's) mercy
  • have (one) by the short and curlies
  • have someone by the short and curlies
  • a stranger to (someone or something)
  • (one) puts (one's) pants on one leg at a time
  • a dog in the manger
  • a happy bunny
  • a happy camper