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

M-expression LISP

   (MLISP) The original "meta-language" syntax of Lisp,
   designed by John McCarthy in 1962.  MLISP was intended for
   external use in place of the parenthesised S-expression
   syntax.

   ["LISP 1.5 Programmer's Manual", J. McCarthy et al, MIT Press
   1962].

   (1994-11-22)