The British have been poking fun at the French for their penchant for eating frog's legs since the 16th century by calling them "frog-eaters," now commonly shortened to just "frogs." The Chinese are known to have eaten frog's legs for millennia, and the Aztecs were partial to them too.
The first mention of them being eaten in Europe was in 12th century annals of the Catholic Church in France. Monks had been ordered to not eat meat for a certain number of days per year and had managed to persuade the church that frogs were fish so that they could eat them. After this, the religiously devout and hungry peasants copied the monk's example, and it became a regular in the history of French cuisine.