Nudity, gore, violence - explicit materials. Stuff you wouldn’t be allowed to plaster on a giant billboards in the middle of the city or on the side of your office building or have run on daytime TV in the breakroom. If an image of a clothed female is NSFW then obviously a man wearing nothing but a towel in a shower is as well.
You start making a list of everything everyone takes offence into and finds inappropriate and you end up with a list with literally everything on it. Some people in this thread have used “If I couldn’t use it as a wallpaper at work, it should be NSFW”. Plenty of people would find this picture absolutely disgusting and inappropriate, so should it and everything like it be NSFW tagged as well?
The etymology follows the drive-in which is basically a big parking lot you drive in to, do your ordering/eating/movie watching in your car, and then you drive out. And when you don’t stop in the middle of a drive in, but instead you continue through it, in your car, it became a drive through.
The pedantic term is a drive-up, btw.