The Collaborative International Dictionary of English v.0.48:

Indorse \In*dorse"\, v. t. [imp. & p. p. Indorsed; p. pr. &
   vb. n. Indorsing.] [LL. indorsare. See Endorse.] [Written
   also endorse.]
   [1913 Webster]
   1. To cover the back of; to load or burden. [Obs.]
      [1913 Webster]

            Elephants indorsed with towers.       --Milton.
      [1913 Webster]

   2. To write upon the back or outside of a paper or letter, as
      a direction, heading, memorandum, or address.
      [1913 Webster]

   3. (Law & Com.) To write one's name, alone or with other
      words, upon the back of (a paper), for the purpose of
      transferring it, or to secure the payment of a note,
      draft, or the like; to guarantee the payment, fulfillment,
      performance, or validity of, or to certify something upon
      the back of (a check, draft, writ, warrant of arrest,
      etc.).
      [1913 Webster]

   4. To give one's name or support to; to sanction; to aid by
      approval; to approve; as, to indorse an opinion.
      [1913 Webster]

   To indorse in blank, to write one's name on the back of a
      note or bill, leaving a blank to be filled by the holder.
      [1913 Webster]

WordNet (r) 3.0 (2006):

indorse
    v 1: be behind; approve of; "He plumped for the Labor Party"; "I
         backed Kennedy in 1960" [syn: back, endorse, indorse,
         plump for, plunk for, support]
    2: give support or one's approval to; "I'll second that motion";
       "I can't back this plan"; "endorse a new project" [syn:
       second, back, endorse, indorse]
    3: guarantee as meeting a certain standard; "certified grade AAA
       meat" [syn: certify, endorse, indorse]
    4: sign as evidence of legal transfer; "endorse cheques" [syn:
       endorse, indorse]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "indorse":
   OK, abet, accede to, accept, accord to, accredit, admire, advocate,
   affirm, agree to, aid and abet, amen, approve, approve of, assent,
   assure, attest, authenticate, authorize, autograph, back, back up,
   be sponsor for, be willing, bless, bond, certify, comfort,
   condescend, confirm, connive at, consent, consent to silently,
   cosign, countenance, countersecure, countersign, deign, elect,
   embolden, encourage, endorse, ensure, esteem, favor, give consent,
   give permission, give the go-ahead, give the imprimatur,
   give thumbs up, go along with, go for, grant, guarantee, guaranty,
   have no objection, hearten, hold with, initial, insure,
   keep in countenance, nod, nod assent, nominate, not refuse,
   notarize, okay, pass, pass on, pass upon, permit, ratify, respect,
   rubber stamp, sanction, say amen to, say aye, say yes, seal,
   second, secure, shine upon, sign, sign and seal, sign for,
   smile upon, sponsor, stand behind, stand up for, subscribe,
   subscribe to, support, swear and affirm, swear to, take kindly to,
   think well of, undersign, underwrite, uphold, validate,
   view with favor, visa, vise, vote, vote affirmatively, vote aye,
   warrant, wink at, yield assent