The Collaborative International Dictionary of English v.0.48:

Thug \Thug\ (th[u^]g; 277), n. [Hind. thag a deceiver, robber.]
   1. 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.
      [1913 Webster]

   2. An assassin; a ruffian; a rough. "Thugs and midnight
      rounders." --The Century.
      [Webster 1913 Suppl.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

thug
    n 1: an aggressive and violent young criminal [syn: hood,
         hoodlum, goon, punk, thug, tough, toughie,
         strong-armer]