NSF56K 

Short for "Not Safe For 56 Kbaud Modems". Tagline abbreviation added to an internet forum post (or other missive) to warn of massive attachment size, which a normal analog phone modem may be incapable of coping with.

Click Here: Britney Spears Sucking Face With Madonna. NSF56K!

NSF56K by 1Spectre4U December 2, 2003

NSF56K 

Not Safe For 56K. A phone line that isn't good enough to get the full connection speed of a 56K modem.

I moved out into the country, and they gave me a NSF56K line!

NSF56K by NSFMR2K December 2, 2003