The word "thug," meaning "a brutal ruffian or assassin," comes to English directly from the Hindi word "thag," which means literally "thief." The earliest known use of "thug" in English is dated 1810, say editors at Merriam Webster Inc.
Originally a thug was a member of a well-organized confederacy of professional assassins in Inda. Their practice was to gain the confidence of travelers, and when an opportunity presented itself, slip a noose or handkerchief around the necks of their victims and strangle them.This procedure was accompanied by some traditional and rigidly prescribed rites. These thugs worshiped the Hindu goddess of destruction, Kali. They also possessed a jargon called "Ramasi" and signs by which members identified themselves to each other.
The organization existed for some 300 years before it was finally quashed in the 19th century.