The Collaborative International Dictionary of English v.0.48:

Undersign \Un`der*sign"\, v. t.
   To write one's name at the foot or end of, as a letter or any
   legal instrument.
   [1913 Webster]

   The undersigned, the person whose name is signed, or the
      persons whose names are signed, at the end of a document;
      the subscriber or subscribers.
      [1913 Webster]