Originally it was petrol, as in gas. But then a guy named Peter sued for copyright infringement (he owned a petrol company), so they changed it to troll.

It was used as a verb long before it was used as a noun. First recorded use as a noun was in 1616, when it meant "ugly dwarf or giant" or "giant, fiend, demon." Some speculate that it originally meant "creature that walks clumsily." See http://www.etymonline.com/index.php?term=troll

It is an acronym....T.R.O.L.L. Terribly Rude Obnoxious Little Loser

Troll, as a noun, comes from Old Norse:(n.) "ugly dwarf or giant," circa 1600; From the Online Etymology Dictionary: "...meaning "giant, fiend, demon." It seems to have been a general supernatural word, Old Norse trolldomr "witchcraft." The old sagas tell of the troll-bull, a supernatural being in the form of a bull, as well as boar-trolls. There were troll-maidens, troll-wives, and troll-women; the trollman, a magician or wizard, and the troll-drum, used in Lappish magic rites. The first record of it is from a court document from the Shetlands, regarding a certain Catherine, who, among other things, was accused of "airt and pairt of witchcraft and sorcerie, in hanting and seeing the Trollis ryse out of the kyrk yeard of Hildiswick." Originally conceived as a race of giants, they have suffered the same fate as the Celtic Danann and are now regarded in Denmark and Sweden as dwarfs and imps supposed to live in caves or under the ground."

Isn't it somewhere in The Hobbit?

All your internet troll related answers can be found here: http://tinyurl.com/yyab85 Wikipedia: The contemporary use of the term first appeared on Usenet groups in the late 1980s. It is widely thought to be a contraction of the phrase trolling for suckers, itself derived from the sport fishing technique of trolling. The latter can be compared with trawling. Another plausible derivation is that it may be a shortening of "patrolling", with the common meaning of "searching," especially, "searching for those who do not wish to be found." The word likely gained currency because of its apt second meaning, drawn from the trolls portrayed in Scandinavian folklore and children's tales; they are often ugly, many headed, obnoxious creatures bent on mischief and wickedness. The image of the troll under the bridge in the "Three Billy Goats Gruff" emphasizes the troll's negative reaction to outsiders intruding on its physical environment, particularly those who intend to graze in its domain without permission. The word occurs also in Thomas Awdeley’s Virginity of Vagabonds (1561) in the form: "Troll and Troll by is he that setteth naught by no man, nor no man by him. This is he that would bear rule in a place and hath no authority nor thanks, and at last is thrust out of the door like a knave." It seems a singularly apt description, though no provenance has ever been demonstrated to connect it with the modern usage.