"punishment by arbitrary or discretionary fine," 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 an adverbial phrase in legalese can become a verb (compare abandon).
Frans hom ne seit amerciez pour petit forfet. [Magna Charta]
Related: Amercement; amerciable/amerceable.