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.