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
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.
To pin one's faith upon, to depend upon; to trust to.