According to the Urban DIctionary,
“Not Safe For Work.” Used to describe Internet content generally inappropriate for the typical workplace, i.e., would not be acceptable in the presence of your boss and colleagues (as opposed to SFW, Safe For Work).
“NSFW” labeled contents can be quite offensive, yes. But it can also just be intriguing, disturbing, or more simply, just not consensual.
We tried to filter the too obvious porn spam by excluding keywords too “explicit”, and hoped for the best.
You’ll find NSFTweets there: http://bienoubien.jp/nsfw/
Alban made the code, and I was responsible for the design and css part.