late 12c., "one who commits robbery, one who steals, plunders, or strips unlawfully by violence," from Anglo-French robbere, Old French robeor, agent noun from rober "to rob, steal, pillage, ransack, rape" (see rob).
Robber baron in the "corrupt, greedy financier" sense is attested from 1870s, from a comparison of Gilded Age capitalists to medieval European warlords (the phrase is attested in the historical sense from 1831).
It is the attempt of the more shrewd to take advantage of the less shrewd. It is the attempt of the strong to oppress the weak. It is the old robber baron in his castle descending, after men have planted their crops, and stealing them. [Henry Ward Beecher, sermon, "Truthfulness," 1871]
Regulation by combination means that the railroad managers are feudal lords and that you are their serfs. It means that every car load of grain or other produce of your fields and shops that passes over the New York Central shall pay heavy toll for right of transit to Vanderbilt, the robber baron of our modern feudalism, who dominates that way. [W.C. Flagg, testimony to Congress, 1874]