Got to love a thread asking at a forum if the word "forum" is consider a "poison word". That is what we got at a thread at WebmasterWorld named Is Forum a Dirty Word?
A poison word is a word that triggers a search engine to "decrease your pages rankings if a search engine finds them in the title, description or in the url. They don't kill, they just bury pages in rankings." (source)
In the current thread, the idea that "forum" is one of those poison words is shut down right away. But if you take a look at a WebmasterWorld thread from 2000 named Poison word Forum which discusses this in greater detail.
I like how tedster put it
I completely agree with martinibuster, "forum" is not a poison word. Just look at WebmasterWorld for the proof.
Forum discussion at WebmasterWorld.