As in, not industrialists but actual medieval nobles who lived by robbing people?
If not, where does the expression actually come from?
Edit: I'm asking mostly because it's difficult to imagine how such a noble wouldn't be dealt with in a fairly short order by his neighbours, if nobody else...
Even though more can be said about it, this answer by /u/A_Soporific goes into the history of the term and describes "real" robber barons.