Other forms: mercies
Mercy is kindness shown to somebody you have the power to punish. A judge shows mercy, a victor shows mercy, and in each case the point is that the harsher option was available and was not taken.
That is what separates it from kindness, which needs no power behind it, and from pity, which is a feeling rather than an act. The word belongs to law, religion and war. Two everyday uses are worth having: at the mercy of something means completely exposed to it, with no protection, and a mercy is a small piece of luck in a bad situation — it is a mercy she was not in the car.
1nounmercyKindness shown to somebody you could punish instead.
“The court showed no mercy at all.”
synonyms: clemency, compassion, leniency
2noungoodA piece of good fortune in a bad situation.
“It was a mercy that the house was empty.”
Too few real examples of this word to estimate which sense is commonest.
mercy is a kind of forbearance.
Latin merces meant wages or a price — the same word behind commerce and merchant. Church Latin turned it into the reward for good works, and then into the pity God grants. So mercy started as something earned and ended as the one thing that cannot be.
Latin
mercēs — wages, fee, price
Anglo-Norman
merci
Middle English
mercy
English, today
mercy
Built from
From Middle English mercy, merci, from Anglo-Norman merci (compare continental Old French merci, mercit), from Latin mercēs (“wages, fee, price”), from merx (“wares, merchandise”). Displaced native mildheortnes (literally "mildheartedness"). Cognate with French merci, whence the doublet a doublet of merci.
Etymology from Wiktionary, CC BY-SA