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

/dev/null
 /dev?nuhl/, n.

    [from the Unix null device, used as a data sink] A notional ?black hole? in
    any information space being discussed, used, or referred to. A
    controversial posting, for example, might end ?Kudos to
    rasputin@kremlin.org, flames to /dev/null?. See bit bucket.


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

/dev/null

    The Unix software device that discards any
   data written to it and supplies zero bytes of data when read.
   /dev/null is occasionally useful in situations where a file is
   required but no relevant file exists.

   For example, if a program produced lots of output for debugging
   purposes but that output was not required when the program was
   deployed in a production environment and writing it to a file
   would waste disk space then the output could be written to
   /dev/null.

   The term is used for a notional "black hole" in any information
   space.  For example, a message, might end, "Kudos to
   rasputin@kremlin.org, flames to /dev/null".

   See bit bucket.

   [Jargon File]

   (2015-01-28)