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1. A disparaging term applied to any of the prominent businessmen in the late 19th-century US who used unethical means to increase industrialization (and thus their own wealth and prestige). John Rockefeller is usually regarded as a robber baron, but he's certainly not the only one.
2. By extension, anyone who achieves great wealth and success by unscrupulous means. Of course I don't trust him—he's a robber baron who's made all his money by manipulating the stock market!
3. In the Middle Ages, a thief who robbed unsuspecting travelers. Please be mindful of robber barons on your journey through the countryside.
See also: robber