All definitions are wrong, which is a very notable achievement, but as other contribtors have already pick up most of them I'll just add that 'Thug' (singular) is derived from the Thuge (or Thuggi) a cult of warriors and assassains in India which has a history stretching back thousands of years. The Thuggee were a kind of proto Sprcial Ops regiment and specialised in carrying out hits on specified targets whom they preferred to kill with bare hands in ritualistic ways. Their lifestyle was ascetic and highly disciplined and thy used Yogic techiques to train themselves to overcome pain and develop preternatural levels of endurance.
There is documentary evidence that 'thug was a colloquial term for violent criminals in 18th century Britain.
Some people on Medium might believe the word Black should be eliminated from the English language but I'm sure the ide would not be popular with the citizens of several towns in northern England, including Blackburn and Blackpool (not to mention Blackrod and Black Dyke (yes, really though it's not a town in its own right)