mercy

Concise Oxford English Dictionary © 2008 Oxford University Press:
mercy/ˈmɜːsi/

▶noun (pl. mercies)

• 1 compassion or forgiveness shown towards an enemy or person in one's power.

• 2 something to be grateful for.

• 3 [as modifier] (especially of a journey or mission) motivated by compassion.

▶exclamation archaic used to express surprise or fear.

– phrases
at the mercy of completely in the power of.

– origin ME: from OFr. merci ‘pity’ or ‘thanks’, from L. merces, merced- ‘reward’, in Christian L. ‘pity, favour, heavenly reward’.