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]

WordNet (r) 3.0 (2006):

undersign
    v 1: sign at the bottom of (a document)

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "undersign":
   OK, accept, accredit, affirm, amen, approve, assure, attest,
   authenticate, authorize, autograph, back, be sponsor for, bond,
   certify, confirm, cosign, countersecure, countersign, endorse,
   ensure, give permission, give the go-ahead, give the imprimatur,
   give thumbs up, guarantee, guaranty, initial, insure, notarize,
   pass, pass on, pass upon, permit, ratify, rubber stamp, sanction,
   say amen to, seal, second, secure, sign, sign and seal, sign for,
   sponsor, stand behind, stand up for, subscribe to, support,
   swear and affirm, swear to, underwrite, validate, visa, vise,
   warrant