In about the 6th century B.C., the Etruscans, led by Lars Porsena, were trying to take Rome. Gaius Mucius was captured trying to assassinate Lars Porsena and brought before the king. He told the king that although he might be executed, there were plenty of other Romans who would try and eventually succeed in assassination. Lars Porsena was angry and threatened to burn Gaius Mucius alive. In response, the Roman stuck his hand in the fire to show he didn't fear it. This so impressed the king, that he sent Gaius Mucius back and made peace with Rome.
Since Gaius Mucius effectively lost his right hand to the fire, he became known as Scaevola.
Source: Livy II