not able to stop sb/sth harming you because they have power or control over you :
I'm not going to put myself at the mercy of the bank.
We were at the mercy of the weather.
not able to stop sb/sth harming you because they have power or control over you :
I'm not going to put myself at the mercy of the bank.
We were at the mercy of the weather.
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005