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.
Some roving robber calling to his fellows. --Milton.
Syn: Thief; depredator; despoiler; plunderer; pillager;
rifler; brigang; freebooter; pirate. See Thief.
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.