Using keywords not associated with a site good or bad?

As Adam pointed out I think it's better to not use keywords that lack relevanancy to your site as that would result in a bad user experience for any visitors that are misled due to such information that is present on your sites metadata or in your content which would result in either a Google penalty or just decrease the domain Authority of your site due to its incredulous use of wrong keywords in an irrelevant website.

I think to some extent you want to make sure the keywords you use are close to what your site is in terms of subject matter. Adding tags/keywords for stuff that doesn't match your site, could hurt, at least I would think so.

Keywords are an important aspect of SEO, and I don't think putting keywords that don't fit helps anything. It may draw some traffic for a bit, but people will see those keywords don't match and might complain.