[ʃˈiʃ. nˈɔ.vɨ] A snitch, a traitor. This slang term is especially common in the state of São Paulo. Believe it or not, it comes from an American comic book from the 1930s.
There are two accepted theories regarding the term X-9. The most popular one suggests that the term comes from the American comic book Secret Agent X-9, first published in 1934 to rival the famous Dick Tracy. It was a creation of writer Dashiell Hammet and artist Alex Raymond (Flash Gordon).
In his adventures, the agent would infiltrate criminal groups by using their slang and imitating their behavior – only to arrest them in the end. So there you go: for criminals, X-9 was always a traitor.
Another explanation evokes the Carandiru Prison, a now-deactivated penitentiary in northern São Paulo where 111 inmates died in a massacre perpetrated by the police. Block 9 used to house first-time offenders. Rumor has it that these men, who had just entered the system, were more susceptible to becoming snitches.
One of the most traditional São Paulo samba schools bears the name X-9. However, the two-time champions of the carnival chose the name after the secret agent.