NSFW

It's a flag on a thread that indicates to readers of a thread's 'not suitable for work' status so people can know in advance that when reading A2K at a job that they could be risking their job if discovered reading certain types of discussion threads. If someone is an under-age reader, that means that an individual may not be welcome on the thread is advised against participation.

Not Safe For Work

Don't click on it if you're at your desk at work. Sexual content implied.

Are you seeing the notation on your own posts with a certain frequency?