1 general:: 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."
American Heritage Idioms