Despite what the letters literally stand for or where it’s from, it doesn’t actually mean that (anymore). If it did for most people almost any media would be “NSFW”. Most people would get in trouble for watching a movie or playing video games at work, regardless of content. That’s obviously a useless definition.
Since it was popularized on reddit and other social media the tag now defacto means porn/gore. That’s how the vast majority of people uses it and that’s how I think it should be used.
And of course people can complain. But well, sometimes the complain might just fall on deaf ears. And in the case if demanding bikini pictures be tagged as NSFW, I think rightfully so.
The literal meaning of the phrase is useless, since what is safe at a workplace varies drastically with the workplace. It’s also relevant in a lot of other settings that aren’t work, like browsing your phone on public transport or at school. It’s also very common for phrases to have a different meaning from the literal meaning of the individual words. I’d say that’s even the case for most phrases. When you say it’s raining cats and dogs there aren’t actually cats and dogs falling from the sky. In this case the actual “work” aspect isn’t relevant.
What the tag does signify is that the following content might be disturbing or inappropriate in certain situations so you can be aware before opening it. And bikini pictures don’t warrant that warning, even if some workplaces have a policy against it.