What's a troll? Internet Vocab FAQ

An internet troll is someone who joins a group or message board with the aim of causing problems. This might be achieved by provoking genuine members of the group with ridiculous, provocative or abusive comments. Sometimes trolling can have a sinister impact Jojo Moyes revealed in a recent article in the Daily Telegraph

Trolling - posting inflammatory comments on web sites is on the rise. Recent victims include a schoolgirl who committed suicide; a reporter attacked in Egypt; and a pregnant celebrity.

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The etymology is complex and can be traced to old French hunting term troller and a norse one for a mythological monster. The interent use of  troll probably derives from a slang term used by US naval pilots in the 1970s - see here.

Internet Slang Dictionary