A common
disclaimer on
internet forums. It indicates whether the link or material cited is
unsavory (
porn,
sick, twisted shit,
deformed babies, etc.), to the extent that would probably be advisable for you not to view while in mixed company, such as at
work, or not. Things which you should avoid while
at work are "not work safe".
The disconcerting implication is that most people who frequent such forums do so while bored at some corporate job, when they are supposed to be working.