dauphin (n.)

title of the eldest son of the king of France (in use from 1349-1830), early 15c., from Old French dauphin, literally "dolphin" (see dolphin).

Originally it was the title attached to "the Dauphin of Viennois," whose province (in the French Alps north of Provence) came to be known as Dauphiné. Three dolphins were on the coat of arms of the lords of Viennois, first worn by Guido IV (d. 1142). It is said originally to have been a personal name among the lords of Viennois. Humbert III, the last lord of Dauphiné, ceded the province to Philip of Valois in 1349, on condition that the title be perpetuated by the eldest son of the king of France. The French fem. form is dauphine.