(Latin). A noisy but harmless threatening; an innocuous thunderbolt.
“His [the Pope's] denunciations are but a brutum fulmen.” - The Standard.
Brutus (Junius ), the first consul of Rome. He condemned to death his own two sons for joining a conspiracy to restore to the throne the banished Tarquin.
The public father [Brutus], who the private quelled, And on the dread tribunal sternly sat.
The Spanish Brutus. Alphonso Perez de Guzman (1258-1320). While he was governor, Castile was besieged by Don Juan, who had revolted from his brother, Sancho IV. Juan, who held in captivity one of the sons of Guzman, threatened to cut his throat unless Guzman surrendered the city. Guzman replied, “Sooner than be a traitor, I would myself lend you a sword to slay him,” and he threw a sword over the city wall. The son, we are told, was slain by the father's sword before his eyes.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894