leave to someone's tender mercies

To allow one to face the punishment or rebukes of another person who will not show them any mercy, kindness, or sympathy. The phrase is used ironically. I thought it would be best if I spoke to her, rather than leaving her to the principal's tender mercies. I don't have time to deal with this. I'll just leave them to Mrs. Tanner's tender mercies.

Submit to another's power or discretion, especially to an unsympathetic individual. Today this expression is always used ironically, as in We left him to the tender mercies of that stiff-necked, arrogant nurse. It alludes to a biblical passage (Proverbs 12:10): "A righteous man regardeth the life of his beast; but the tender mercies of the wicked are cruel."

Literally, kind usage of someone. The phrase “tender mercies” comes from the Bible’s Book of Psalms (25:6), “Remember, O Lord, thy tender mercies and they loving kindnesses.” Since at least 1900 or so the term has been used ironically, as in Stella Rimington’s At Risk (2004): “‘What would you have done if the guy had refused to give back the money?’ ‘Left him to your tender mercies,’ said Liz. . . . ‘We don’t do violence.’”