The Collaborative International Dictionary of English v.0.48:

Robber \Rob"ber\, n.
   One who robs; in law, one who feloniously takes goods or
   money from the person of another by violence or by putting
   him in fear.
   [1913 Webster]

         Some roving robber calling to his fellows. --Milton.
   [1913 Webster]

   Syn: Thief; depredator; despoiler; plunderer; pillager;
        rifler; brigang; freebooter; pirate. See Thief.
        [1913 Webster]

   Robber crab. (Zool.)
   (a) A purse crab.
   (b) Any hermit crab.

   Robber fly. (Zool.) Same as Hornet fly, under Hornet.


   Robber gull (Zool.), a jager gull.
      [1913 Webster]

WordNet (r) 3.0 (2006):

robber
    n 1: a thief who steals from someone by threatening violence

The Devil's Dictionary (1881-1906):

ROBBER, n.  A candid man of affairs.
    It is related of Voltaire that one night he and some traveling
companion lodged at a wayside inn.  The surroundings were suggestive,
and after supper they agreed to tell robber stories in turn.  "Once
there was a Farmer-General of the Revenues."  Saying nothing more, he
was encouraged to continue.  "That," he said, "is the story."