Source : Webster's Revised Unabridged Dictionary (1913)

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

   {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.

Source : WordNet®

undersign
     v : sign at the bottom of (a document, for example)