The old Snopes forums used to use NSFBSK, Not Safe for British School Kids, after a time when a bunch of, well, BSK signed up and there was a bit of a kerfuffle regarding adult content and separating it from general boards
On the other hand, it's coming from an era where people on the internet assumed the only reason someone wouldn't want to look at porn, is that they were at work.
I think a lot of it goes WAY back to when people mostly got on the internet from work and the intent was "don't click this link or you might get in trouble," but yeah we should have moved on from that by now it's weird and its use is so far removed from that context
It really sucks as a euphemism for porn. Which is unfortunate because NSFW is actually an extremely useful term when used as intended, at the scope it was intended for (which goes beyond just porn)
The olds remember when they tried to give the websites movie ratings. (Remember the Censorship Pandas?) But I guess what actually *stuck* as a concept was "this will get you in trouble with HR".