The Free On-line Dictionary of Computing (19 January 2023):

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)


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

RSN
 /R·S·N/, adj.

    See Real Soon Now.