The Collaborative International Dictionary of English v.0.48:

Apron string \A"pron string`\
   The string of an apron.
   [1913 Webster]

   To be tied to a wife's apron strings or To be tied to a
   mother's apron strings, to be unduly controlled by a wife or
      mother.
      [1913 Webster]

            He was so made that he could not submit to be tied
            to the apron strings even of the best of wives.
                                                  --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

apron string
    n 1: (usually used in the plural) a cord used to tie an apron at
         the waist