DuckCorp Dico

Found one definition

1.                 From The Collaborative International Dictionary of English v.0.48:
                   
   
   
                   
                     
   arb \arb\ n. [Shortened from arbitrageur.]
      Someone who engages in arbitrage (who purchases securities in
      one market for immediate resale in another in the hope of
      profiting from the price differential); shortened form of
      arbitrageur.
      Syn: arbitrageur
           [WordNet 1.5] Arbalest