thug

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A cutthroat or ruffian; a hoodlum.
• noun One of a group of professional criminals, devotees of Kali, who robbed and murdered travelers in northern India until the mid-1800s.

from The Century Dictionary.

• noun A member of a confraternity of professional assassins and robbers formerly infesting India, chiefly in the central and northern provinces.
• noun Hence A cutthroat; a ruffian; a rough.

from the GNU version of the Collaborative International Dictionary of English.

• noun One of an association of robbers and murderers in India who practiced murder by stealthy approaches, and from religious motives. They have been nearly exterminated by the British government.
• noun An assassin; a ruffian; a rough.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A criminal with intimidating mannerisms, who treats others violently and roughly, especially for hire.
• noun dated One of a band of assassins formerly active in northern India who worshipped Kali and offered their victims to her.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an aggressive and violent young criminal