Mercy  —n. (pl. -ies) 1 compassion or forbearance towards defeated enemies or offenders or as a quality. 2 act of mercy. 3 (attrib.) Done out of compassion (mercy killing). 4 thing to be thankful for (small mercies). —int. Expressing surprise or fear.  at the mercy of 1 in the power of. 2 liable to danger or harm from. Have mercy on (or upon) show mercy to. [latin merces reward, pity]