The Collaborative International Dictionary of English v.0.48:

Underwrite \Un`der*write"\, v. t. [imp. Underwrote, Obs.
   Underwrit; p. p. Underwritten, Obs. Underwrit; p. pr. &
   vb. n. Underwriting.]
   1. To write under something else; to subscribe.
      [1913 Webster]

            What addition and change I have made I have here
            underwritten.                         --Bp.
                                                  Sanderson.
      [1913 Webster]

   2. To subscribe one's name to for insurance, especially for
      marine insurance; to write one's name under, or set one's
      name to, as a policy of insurance, for the purpose of
      becoming answerable for loss or damage, on consideration
      of receiving a certain premium per cent; as, individuals,
      as well as companies, may underwrite policies of
      insurance. --B. Jonson.
      [1913 Webster]

            The broker who procures the insurance ought not, by
            underwriting the policy, to deprive the parties of
            his unbiased testimony.               --Marshall.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Underwrite \Un`der*write"\, v. i.
   To practice the business of insuring; to take a risk of
   insurance on a vessel or the like.
   [1913 Webster]

WordNet (r) 3.0 (2006):

underwrite
    v 1: guarantee financial support of; "The opera tour was
         subvented by a bank" [syn: underwrite, subvention,
         subvent]
    2: protect by insurance; "The insurance won't cover this" [syn:
       cover, insure, underwrite]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "underwrite":
   OK, accede to, accept, accredit, affirm, agree to, amen, approve,
   arm, armor, assure, attest, authenticate, authorize, autograph,
   avouch, back, back up, be sponsor for, bless, bond, certify,
   champion, cloak, compass about, confirm, consent to, copyright,
   cosign, countersecure, countersign, cover, cushion, defend,
   endorse, ensure, fence, fend, finance, give permission,
   give the go-ahead, give the imprimatur, give thumbs up, guarantee,
   guaranty, guard, harbor, haven, indorse, initial, insure,
   invest in, keep, keep from harm, make a promise, make safe, nestle,
   notarize, okay, pass, pass on, pass upon, patent, permit, pledge,
   plight, police, promise, protect, ratify, register,
   ride shotgun for, rubber stamp, safeguard, sanction, say amen to,
   screen, seal, second, secure, shelter, shield, shroud, sign,
   sign and seal, sign for, sponsor, stand behind, stand up for,
   subscribe to, subsidize, subvene, support, swear, swear and affirm,
   swear to, troth, undersign, uphold, validate, visa, vise, vouch,
   vow, warrant