The term is often used in conjunction with bad neighborhood. It is recommendable to dispense with bad keywords because since the Google Penguin update, a webpage containing such will be devalued if the Google search engine identifies them while crawling.
When talking about bad keywords, SEOs refer to two things:
Generally, it is recommended to use common sense when configuring a domain or in the preparation of content for HTML pages. It is a good idea to take on the viewpoint of a user and then select good keywords, i.e. keywords with lots of conversion potential. The premise is: optimize with carefully selected keywords, increase the conversion rates of the keywords entered by users, and maintain it at a high level. This also includes regular keyword analysis to make improvements.
Keyword combinations are also an important factor with respect to a high conversion rate, since users who know exactly what they are looking for will enter several terms in the search bar. That way, they narrow down the results and find suitable answers quickly. In SEO we speak of so-called long tail combinations. This can be two or three terms and sometimes even whole sentences or questions. Through a mix of main keywords (also called money keys) and longtail combinations, a wide range of search volume can be covered. Finally, there are per se no bad keywords, only poor technical handling of them.