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.
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   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]

The Collaborative International Dictionary of English v.0.48:

Endorse \En*dorse"\, v. t. [imp. & p. p. Endorsed; p. pr. &
   vb. n. Endorsing.] [Formerly endosse, fr. F. endosser to
   put on the back, to endorse; pref. en- (L. in) + dos back, L.
   dorsum. See Dorsal, and cf. Indorse.]
   Same as Indorse.
   [1913 Webster]

   Note: Both endorse and indorse are used by good writers; but
         the tendency is to the more general use of indorse and
         its derivatives indorsee, indorser, and indorsement.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Endorse \En*dorse"\, n. (Her.)
   A subordinary, resembling the pale, but of one fourth its
   width (according to some writers, one eighth).
   [1913 Webster]

WordNet (r) 3.0 (2006):

endorse
    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]