mercy flight noun

An aircraft flight taking a seriously ill or injured person to hospital when other means of transport are impracticable or unavailable

mercy killing noun

Killing, esp painlessly, to prevent chronic unrelenting suffering

merˈcy-seat noun

1. The seat or place of mercy
2. The covering of the Jewish Ark of the Covenant
3. The throne of God

at the mercy of

Wholly in the power of

for mercy (obsolete) or for mercy's sake

An earnest conjuration in the form of an appeal to pity

leave (a person) to someone's tender mercies or mercy (ironic)

To leave (a person) exposed to unpleasant treatment at someone's hands

sisters of mercy

Members of female religious communities who tend the sick, etc