The Collaborative International Dictionary of English v.0.48:

Mother \Moth"er\, a.
   Received by birth or from ancestors; native, natural; as,
   mother language; also acting the part, or having the place of
   a mother; producing others; originating.
   [1913 Webster]

         It is the mother falsehood from which all idolatry is
         derived.                                 --T. Arnold.
   [1913 Webster]

   Mother cell (Biol.), a cell which, by endogenous divisions,
      gives rise to other cells (daughter cells); a parent cell.


   Mother church, the original church; a church from which
      other churches have sprung; as, the mother church of a
      diocese.

   Mother country, the country of one's parents or ancestors;
      the country from which the people of a colony derive their
      origin.

   Mother liquor (Chem.), the impure or complex residual
      solution which remains after the salts readily or
      regularly crystallizing have been removed.

   Mother queen, the mother of a reigning sovereign; a queen
      mother.

   Mother tongue.
   (a) A language from which another language has had its
       origin.
   (b) The language of one's native land; native tongue.

   Mother water. See Mother liquor (above).

   Mother wit, natural or native wit or intelligence.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mother tongue
    n 1: one's native language; the language learned by children and
         passed from one generation to the next [syn: mother
         tongue, maternal language, first language]