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

rare mode
 adj.

    [Unix] CBREAK mode (character-by-character with interrupts enabled).
    Distinguished from raw mode and cooked mode; the phrase ?a sort of
    half-cooked (rare?) mode? is used in the V7/BSD manuals to describe the
    mode. Usage: rare.


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

rare mode

    Unix CBREAK mode (character-by-character
   with keyboard interrupts enabled).  Distinguished from raw
   mode and cooked mode; the phrase "a sort of half-cooked
   (rare?) mode" is used in the V7/BSD manuals to describe the
   mode.  Usage: rare.

   [Jargon File]

   (1995-05-10)