The Collaborative International Dictionary of English v.0.48:

Mun \Mun\, n. [See Mouth.]
   The mouth. [Obs.]
   [1913 Webster]

         One a penny, two a penny, hot cross buns,
         Butter them and sugar them and put them in your muns.
                                                  --Old Rhyme.
                                                  --Halliwell.
   [1913 Webster]