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1215, earlier amercy, Anglo-French amercier "to fine," from merci "mercy, grace" (see mercy). The legal phrase estre a merci "to be at the mercy of" (a tribunal, etc.) was corrupted to estre amercié in an example of how a legalese adverbial phrase can become a verb (cf. abandon). The sense often was "to fine arbitrarily."
Frans hom ne seit amerciez pour petit forfet. [Magna Charta]Related: Amercement; amerciable.
in English law, an arbitrary financial penalty, formerly imposed on an offender by his peers or at the discretion of the court or the lord. Although the word has become practically synonymous with "fine," there is a distinction in that fines are fixed by statute, whereas amercements are decided by the court. Originally, an amercement represented a commutation of a sentence that required the forfeiture of goods, while a fine was an arrangement agreed upon between the judge and the prisoner to avoid imprisonment. Magna Carta (1215) attempted to regulate the assessment of amercements.