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

inout

    A type or "mode" of function parameter that
   passes information in both directions - from the caller to the
   function and back to the caller, combining the in and out
   modes.  An "inout" parameter might be used where the function
   needs to read and update some data belonging to the caller as
   a side effect of its main purpose.

   (2010-01-19)