robber baron

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One of the American industrial or financial magnates of the late 19th century who became wealthy by unethical means, such as questionable stock-market operations and exploitation of labor.
2. n. A feudal lord who robbed travelers passing through his domain.

Wiktionary

1. n. historical In Europe, an aristocrat who charged exorbitant fees or otherwise exacted money from people who journeyed across land or waterways which he controlled.
2. n. idiomatic, usually derogatory Especially in the 19th-century and early 20th-century, a business tycoon who had great wealth and influence but whose methods were morally questionable.