The Collaborative International Dictionary of English v.0.48:

Promisor \Prom"is*or\, n. (Law)
   One who engages or undertakes; a promiser. --Burrill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

promisor
    n 1: a person who makes a promise [syn: promiser, promisor]

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

PROMISOR. One who makes a promise.
     2. The promisor is bound to fulfill his promise, unless when it is
contrary to law, as a promise to steal or to commit an assault and battery;
when the fulfillment is prevented by the act of God, as where one has agreed
to teach another drawing and he loses his sight, so that he cannot teach it;
when the promisee prevents the promisor from doing what he agreed to do;
when the promisor has been discharged from his promise by the promisee, when
the promise, has been made without a sufficient consideration; and, perhaps,
in some other cases, the duties of the promisor are at an end.