merciful adj.

late 12c., "God's forgiveness of his creatures' offenses," from Old French mercit, merci (9c.) "reward, gift; kindness, grace, pity," from Latin mercedem (nominative merces) "reward, wages, pay, hire" (in Vulgar Latin "favor, pity;" in Medieval Latin "thanks; grace"), from merx (genitive mercis) "wares, merchandise" (see market (n.)). In Church Latin (6c.) it was given a specific application to the heavenly reward earned by those who show kindness to the helpless and those from whom no requital can be expected.

Meaning "disposition to forgive or show compassion" is attested from early 13c. Sense of "an act or exercise of forbearance or good will" is from c. 1300. As an interjection, attested from mid-13c. (short for may God have mercy, have mercy on me, etc.).  Many of the English senses are found earlier in French, but in French the word largely has been superseded by miséricorde except as a word of thanks. Sense of "discretionary action" (as in at (one's) mercy) is from mid-14c. Seat of mercy "golden covering of the Ark of the Covenant" (1530), hence "the throne of God," is Tyndale's loan-translation of Luther's gnadenstuhl, an inexact translation of Latin propitiatorium, ultimately a rendering of Hebrew kapporeth, literally "propitiatory."

word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.

It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).

late 15c., from un- (1) "not" + merciful (adj.). Related: Unmercifully.