E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The ancient palace of the Laterani, given by the Emperor Constantine to the popes. Lateran, from lateo, to hide, and rana, a frog. It is said that Nero on one occasion vomited a frog covered with blood, which he believed to be his own progeny, and had it hidden in a vault. The palace which was built on the site of this vault was called the Lateran, or the palace of the hidden frog. (Buckle: History of Civilisation.)
The locality in Rome so called contains the Lateran palace, the Piazza, and the Basilica of St. John Lateran. The Basilica is the Popes cathedral church. The palace (once a residence of the popes) is now a museum.