The Collaborative International Dictionary of English v.0.48:

Pledge \Pledge\, v. t. [imp. & p. p. Pledged; p. pr. & vb. n.
   Pledging.] [Cf. OF. pleiger to give security. See Pledge,
   n.]
   1. To deposit, as a chattel, in pledge or pawn; to leave in
      possession of another as security; as, to pledge one's
      watch.
      [1913 Webster]

   2. To give or pass as a security; to guarantee; to engage; to
      plight; as, to pledge one's word and honor.
      [1913 Webster]

            We mutually pledge to each other our lives, our
            fortunes, and our sacred honor.       --The
                                                  Declaration of
                                                  Independence.
      [1913 Webster]

   3. To secure performance of, as by a pledge. [Obs.]
      [1913 Webster]

            To pledge my vow, I give my hand.     --Shak.
      [1913 Webster]

   4. To bind or engage by promise or declaration; to engage
      solemnly; as, to pledge one's self.
      [1913 Webster]

   5. To invite another to drink, by drinking of the cup first,
      and then handing it to him, as a pledge of good will;
      hence, to drink the health of; to toast.
      [1913 Webster]

            Pledge me, my friend, and drink till thou be'st
            wise.                                 --Cowley.
      [1913 Webster]