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!
by 1Spectre4U December 02, 2003
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!
by NSFMR2K December 02, 2003

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