The Forty Thieves — likely named after Ali Baba and the Forty Thieves — were formed in 1825 and alleged to be the first known and oldest New York City criminal street gang. The Thieves consisted primarily of Irish immigrants and Irish Americans who terrorized the Five Points neighborhood of 19th century Manhattan. Another criminal gang named the "Forty Thieves" which had no criminal ties to the New York gang was formed in London, England in 1828. From 1873-1950s, an all-female London criminal gang known as the "Forty Elephants" was also known to use the name the Forty Thieves. Later a criminal gang in Philadelphia called themselves the Forty Thieves. The Kerryonians, another early Irish gang formed in the same year as the Forty Thieves, have been alleged to be the second oldest organized criminal gang in New York City.

A gang is a group or society of associates, friends, or members of a family with a defined leadership and internal organization that identifies with or claims control over territory in a community and engages, either individually or collectively, in illegal, and possibly violent, behavior, with such behavior often constituting a form of organized crime.