The Collaborative International Dictionary of English v.0.48:

Black Hand \Black Hand\ [A trans. of Sp. mano negra.]
   1. A Spanish anarchistic society, many of the members of
      which were imprisoned in 1883.
      [Webster 1913 Suppl.]

   2. A lawless or blackmailing secret society, esp. among
      Italians. [U. S.]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Black Hand
    n 1: a secret terrorist society in the United States early in
         the 20th century