Robber, 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. (Zoöl.) (a) A purse crab. (b) Any hermit crab. -- Robber fly. (Zoöl.) Same as Hornet fly, under Hornet. -- Robber gull (Zoöl.), a jager gull.
Shades of meaning
A robber seeks to obtain the property of others by force or intimidation; a thief by stealth and secrecy. In early English thief was freely used in both senses, as in Shakespeare and the Authorized Version of the English Bible, which has "two thieves" (Matt. xxvii, 38), where the Revised Version more correctly substitutes "two robbers."