c. 1300, exclamation of thanks, later of surprise, from Old French grant-merci, gran merci "great thanks, many thanks," from gran (see grand (adj.)) + merci "reward, favor, thanks" (see mercy (n.)). Modern French merci "thank you" is a shortening of this.
New York City's Gramercy Park is named for the Gramercy Farm which once stood there; the first part of the name is an 18c. folk-etymology from Crommeshie Fly, the name of a former marsh or shallow pond that stood nearby, itself a mangling of New Netherlands Dutch Crommessie Vly, the first part of which represents either *Krom Moerasje "little crooked swamp" or *Krom Messje "little crooked knife," said to have been the name of a brook flowing into (or out of) the pond.