The Collaborative International Dictionary of English v.0.48:

Allonge \Al*longe"\, n. [F. allonge, earlier alonge, a
   lengthening. See Allonge, v., and cf. Lunge.]
   1. (Fencing) A thrust or pass; a lunge.
      [1913 Webster]

   2. A slip of paper attached to a bill of exchange for
      receiving indorsements, when the back of the bill itself
      is already full; a rider. [A French usage] --Abbott.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Allonge \Al*longe"\, v. i. [F. allonger; [`a] (L. ad) + long (L.
   longus) long.]
   To thrust with a sword; to lunge.
   [1913 Webster]

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

ALLONGE, French law. When a bill of exchange, or other paper, is too small
to receive the endorsements which are to be made on it, another piece of
paper is added to it, and bears the name of allonge. Pard. n. 343; Story on
P. N. Sec. 121, 151; Story on Bills, 204. See Rider.