Thug is a term for a violent, lawless person, especially a man.
The word was originally used in the 1800s as a name for members of a group of men in India said to be professional criminals and murderers.
Since then, thug has come to have a more general meaning similar to its more old-fashioned synonym ruffian. The act of behaving as a thug is known as thuggery. In this sense, thug often refers to someone who acts as a bully or is a professional and violent criminal, as in:
"The mafia sent hired thugs to intimidate store owners."