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

[incr Tcl]

    An extension of Tcl that adds classes and
   inheritence.

   The name is a pun on C++ - an object-oriented extension of
   C - [incr variable] is the Tcl syntax for adding one to a
   variable.

   [Origin?  Availability?]

   (1998-11-27)