This article is from the alt.usage.english FAQ, by Mark Israel misrael@scripps.edu with numerous contributions by others.
This word, meaning "to fish by trailing bait behind one's boat",
and hence "to post an article to Usenet designed to elicit flames
from new or unperceptive readers, while signalling levity to the
savvy and experienced", is unconnected with "to trawl" (="to fish
by dragging a net along the sea floor"). "Troll" seems to come
from Middle French "troller"="to run here and there", of Germanic
origin, cognate with Middle High German "trollen"="to walk or run
with short steps" and perhaps also with "troll", the mythological
being; "trawl" seems to come from Middle Dutch "traghel"="dragnet",
perhaps from Latin "tragula"="dragnet", from "trahere"="drag".
 
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