Poison words are those words because of which Search Engines will decrease your ranking if found in your URL, Title, or Description. This can be a disaster when it comes to ranking and therefore you should be very careful and avoid them.
Poison Words Can be Divided into three categories-
Ranking Killers (Adult Words and Obscenities)
Use of these words will cause the major Search Engines to analyze the website as “Adult”. For a mainstream website this can mean your visitors never find your site. If one of these words is necessary, its always best to star out one of the letters. The most popular forum software and CMS systems will allow you to do this through their Admin panels.
These include words the the major Search Engines associate with a lower quality site. These includes but are not limited to Links, Search Engine, Bookmarks, Resources, Directory, BBS, Paid-to-Surf and Forum. The pages containing these words will still be indexed however they will find it very difficult to rank for their main keywords.
These words are similar to Ranking Decreasers except they are merely theory at this point. They include words like “Free” or “Offer”. Many of the pages using these words seemed to take a hit in the Bourbon Update.
The moral of the story is that you should be very careful with your Keyword choices, especially when writing your title and description. If you have a forum or a CMS system, use the profanity filters whenever possible
We can also make some assumptions on Google in particular by referencing the stop words used by their AdSense PPC program.