A patent troll is a pejorative term for someone who buys or registers over-broad patents then sues for ownership.
A patent troll uses patents as legal weapons, instead of actually creating any new products or coming up with new ideas. Instead, trolls are in the business of litigation (or even just threatening litigation). They often buy up patents cheaply from companies down on their luck who are looking to monetize what resources they have left, such as patents. Unfortunately, the Patent Office has a habit of issuing patents for ideas that are neither new nor revolutionary, and these patents can be very broad, covering everyday or commonsense types of computing – things that should never have been patented in the first place. Armed with these overbroad and vague patents, the troll will then send out threatening letters to those they argue infringe their patent(s). These letters threaten legal action unless the alleged infringer agrees to pay a licensing fee, which can often range to the tens of thousands or even hundreds of thousands of dollars.
When do you ask, did this practice develop? Just after 1687.