What's a troll? Internet Vocab FAQ
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