The Collaborative International Dictionary of English v.0.48:

Wishing \Wish"ing\,
   a. & n. from Wish, v. t.
   [1913 Webster]

   Wishing bone. See Wishbone.

   Wishing cap, a cap fabled to give one whatever he wishes
      for when wearing it.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wishing cap
    n 1: a magical cap that secures whatever one wishes for