The Collaborative International Dictionary of English v.0.48:

Pin \Pin\, v. t. [imp. & p. p. Pinned; p. pr. & vb. n.
   Pinning.] [See Pin, n.]
   To fasten with, or as with, a pin; to join; as, to pin a
   garment; to pin boards together. "As if she would pin her to
   her heart." --Shak.
   [1913 Webster]

   To pin one's faith upon, to depend upon; to trust to.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pinning
    n 1: a mutual promise of a couple not to date anyone else; on
         college campuses it was once signaled by the giving of a
         fraternity pin