NSFW

NSFW means Not Safe For Work.

NSFW also includes schools, naturally enough.

If it is NSFW, you should clearly label this fact, because people tend to not want to see things that a NSFW while they are at work or school because it can get them into trouble.

SFW, on the other hand, means that it is indeed Safe for Work.

NSFW's close friend is NSFB.