Thug began as an Indian word for a con man, and for a long time it meant something far narrower than it does now.
The source is Hindi and Urdu thag, "thief" or "swindler." Etymonline says it is possibly from Sanskrit sthagayati, "covers, conceals," a verb from the same ancient root as English deck, thatch and detect. The idea is hiding something under a cover. A thag was someone who hid his purpose behind a friendly face.
English picked the word up in the early 1800s. Etymonline dates its first English use to 1810, and it meant a member of a gang of robbers and killers in India. The gangs were said to befriend travelers on the great highways and then strangle them. NPR adds that they reportedly killed thousands of people.
The word became famous in the 1830s. Lord William Bentinck and Captain William Sleeman led a campaign against the "Thugs," and NPR reports that nearly 4,000 were identified and about 2,000 convicted. Britannica gives different figures: 3,266 captured between 1831 and 1837, of whom 412 were hanged. The counts come from British records and differ by source. Sleeman later declared the system destroyed.
Not every historian accepts the colonial account. Some argue that the cult was largely a British invention that helped justify tighter control of India. NPR cites Martine van Woerkens, author of The Strangled Traveler, who suggests the cult may have resulted from "colonial imaginings." The crimes were real. The size and the ritual structure of the organization are what remain debated.
By 1839 the word had shed its Indian setting. Merriam-Webster records that year for the general sense of a violent or brutish criminal, and Etymonline adds the derived forms thuggery (1839) and thuggish (1870). The capital letter went and so did the strangling.
More recently the word has been reclaimed. Merriam-Webster notes that since the early 1990s rap artists have given it a more nuanced, sometimes positive meaning, though the criminal sense remains strong.
So a word for a deceiver who concealed his intent became, in two centuries, a general insult, and then a badge.