Who was Brutus the Stupid, and where did he get his unusual nickname?

"Brutus the Stupid" is redundant, since "brutus" in Latin can mean dull or slow-witted.

Lucius Junius Brutus (/?lu??i?s, -??s, ?d?u?nj?s ?bru?t?s/) was the founder of the Roman Republic and traditionally one of the first consuls in 509 BC. He was claimed as an ancestor of the Roman gens Junia, including Decimus Junius Brutus and Marcus Junius Brutus, the most famous of Julius Caesar's assassins.

https://en.wikipedia.org/wiki/Lucius_Junius_Brutus

Lucius Junius Brutus was ancient Roman hero. The son of Marcus Junius by the daughter of the elder Tarquin. He saved his life from the persecutions of Tarquin the Proud by feigning himself insane, whence his name Brutus (stupid).

https://medium.com/@EncounterBooks/don-t-judge-a-man-by-his-nickname-brutus-and-the-founding-of-the-roman-republic-f38798c51d6a#.ewdafhmoi

I was referring to the Brutus in lanfranco's answer, who played a key role in Roman hisory.