A metatroll is an internet troll who trolls by deliberately provoking or 'feeding' other internet trolls, attempting to make them believe that people are reacting seriously to the original troll attempt, or attempting to provoke other people into believing the metatroll has fallen for the original troll when in fact the metatroll has not done so.
Troll on Yahoo! Answers: 'OMG I just sat on the baby I was babysiting and broke its neck how do I dispose of the body Casey Anthony style!!1?1'
Metatroll: 'Ur an awful humna beingg and I have reported this to the policce, the CIA, and the UN!'
Original troll: 'NOO but it was totes non-deliberate!!'
And so on.