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
The locality in Rome so called contains the Lateran palace, the Piazza, and the Basilica of St. John Lateran. The Basilica is the Pope's cathedral church. The palace (once a residence of the popes) is now a museum.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894