The Collaborative International Dictionary of English v.0.48:

Indorser \In*dors"er\, Indorsor \In*dors"or\, n.
   The person who indorses. [Written also endorser.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

indorser
    n 1: someone who expresses strong approval [syn: subscriber,
         endorser, indorser, ratifier]
    2: a person who transfers his ownership interest in something by
       signing a check or negotiable security [syn: endorser,
       indorser]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

INDORSER, contracts. The person who makes an indorsement.
     2. The indorser of a bill of exchange, or other negotiable paper, by
his indorsement undertakes to be responsible to the holder for the amount of
the bill or note, if the latter shall make a legal demand from the payer,
and, in default of payment, give proper notice thereof to the indorser. But
the indorser may make his indorsement conditional, which will operate as a
transfer of the bill, if the condition be performed; or he may make it
qualified, so that he shall not be responsible on non-payment by the payer.
Chitty on Bills, 179,180.
     3. To make an indorser liable on his indorsement, the instrument must
be commercial paper, for the indorsement of a bond or single bill.will not,
per se, create a responsibility. 13 Serg. & Rawle, 311. But see Treval v.
Fitch, 5 Whart. 325; Hopkins v. Cumberland Valley R. R. Co., 3 Watts & Serg.
410.
     4. When there are several indorsers, the. first in point of time is
generally, but not always, first-responsible; there may be circumstances
which may cast the responsibility, in the first place, as between them, on a
subsequent indorsee. 5 Munf. R. 252.