mercy: [12] Latin mercēs meant ‘payment, reward’. In the Christian era the notion of a ‘reward’ was taken up and reapplied metaphorically to the ‘compassion given freely by God to humankind’, and the word passed into Old French (in the form merci) with the broader sense ‘compassion’, and hence ‘forbearance from punishment’. English took it over and has continued to use it in much the same way, but its main role in modern French is as the word for ‘thank you’. [mercy etymology, mercy origin, 英语词源]