quoth the OED:

The post-classical uses of merces are developed from the specific application of the word to the reward in heaven which is earned by kindness to those who have no claim, and from whom no requital can be expected. The Eng. uses explained below represent OF. senses that for the most part have not survived in Fr., where the word has been in great part superseded by miséricorde. The chief uses of merci in mod.Fr. are in the sense ‘thanks’ (cf. gramercy), and in phrases corresponding to those in 5 below. {to do mercy, at mercy, at the mercy of, etc.}