The Jargon File (version 4.4.7, 29 Dec 2003):

Real Soon Now
 adv.

    [orig. from SF's fanzine community, popularized by Jerry Pournelle's column
    in BYTE]

    1. Supposed to be available (or fixed, or cheap, or whatever) real soon now
    according to somebody, but the speaker is quite skeptical.

    2. When one's gods, fates, or other time commitments permit one to get to
    it (in other words, don't hold your breath). Often abbreviated RSN. Compare
    copious free time.


The Free On-line Dictionary of Computing (30 December 2018):

Real Soon Now
RSN

    (RSN) A phrase used ironically when you believe
   an event will take a long or unknown time to occur.  The term
   originated in SF's fanzine community, popularised by Jerry
   Pournelle's column in BYTE.

   The phrase can be used, for example, when a manager asks how long
   it will take you to debug some software and you have no idea.
   "I'll have it working Real Soon Now."

   [Jargon File]

   (2013-08-22)