NSFW is Internet slang that means "not safe for work". Another similar acronym is NWS which stands for "not work safe". It is typically used on interactive discussion areas to mark URLs that may be pornographic or shocking, so that the reader can avoid content that may be objectionable to coworkers or put their job at risk where there is a company policy of scanning access logs.
Darla_Crane: I love this...but be warned, this is link is GAY DRAG PORN from the late 1800s. You've been warned. =) http://t.co/8YgJ9B4h #NSFW
Pornific: Free NSFW Porno Video: Blond angel for you http://t.co/jyLdyhjd #porn #videos #xxx #sex #teen #hardcore #cum #pornstar #tits #pussy
Favstar_NSFW: RT @JoshMalina: It's a shame Rod Serling died before writing the Twilight Zone episode where my wife finally caps our toothpaste and the fucking world ends.