Poison words, or forbidden words, is the name given to words or phrases that trigger suspicion, mistrust and loss of respect, or are of inappropriate character for a given web site in its consideration for a search engine.
There is no definite list of poison words which all natural language processing tools incorporate.
This is different from harmless but useless words that are called stop words.
Adult (obscene) words can put a web page in an adult category where it is filtered out by various filters at search engines, so this is one set of poison words. But some consider any words that lower the ranking in a search engine as poison words. Some people consider any words that encourage ads to pervade a whole site and displace much higher earning ads as poison words.