From my understanding trolling meant exactly what it says it is: Trolling. I think people for some reason get this mixed up with trolls - as in the fantasy type monster. But I think it actually has to do with the fishing termtrolling where you cast out your line, and see if you can get somebody to take the bait. Once they take the bait, you take em for a ride.

When the word is used on the Internet it’s meant in the fantasy monster way. Specifically it comes from the story of the troll underneath the bridge, interfering with people trying to cross the bridge.