Not safe for 56K. Similiar to NSFW, NSF56K means that if you are connected to the internet via dialup, don't click on the link. Unless you have a lot of patience.
"Not safe for 56k." This term is used for something on the web that people on slow connections shouldn't mess with. Typically, this includes large images, video, or streams.
Web sites with large pictures often display NSF56K meaning "Not Safe/Suitable for 56K". Contrast with NSFW, "Not Safe/Suitable for work", used to mark questionable content should your boss be shoulder surfing.
A label on a link that will load a heinously huge web page or file. Said page/file will finish loading around the apocalypse on a 56k dial-up connection.
a notation, usually in a forum thread title, warning users of 56k phoneline modems of large pictures or other such files, which result in slow loading times
Not Safe For 56K: containing more data than is sane to download via 56k modem. From "NSFW", relating to items that should only be looked at in a work environment, especially if the office is open-plan.