• WordNet 3.6
    • n promisor a person who makes a promise
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Webster's Revised Unabridged Dictionary
    • n Promisor (Law) One who engages or undertakes; a promiser.
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Century Dictionary and Cyclopedia
    • n promisor In law, one who promises.
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In literature:

The natural question is, what the promisor was to have for his promise.
"The Common Law" by Oliver Wendell Holmes, Jr.
Very often other persons, called fidejussors or sureties, are bound for the promisor, being taken by promises as additional security.
"The Institutes of Justinian" by Caesar Flavius Justinian
If he guarantees "the payment of the note," he is generally considered liable as an original promisor.
"The Government Class Book" by Andrew W. Young
With us, a verbal promise is, generally speaking, to be gathered exclusively from the words of the promisor.
"Ancient Law" by Sir Henry James Sumner Maine
Again, it need not be anything which is obviously for the promisor's benefit.
"Encyclopaedia Britannica, 11th Edition, Volume 7, Slice 2" by Various
When form and intention concurred the promisor must answer for what he undertook.
"An Introduction to the Philosophy of Law" by Roscoe Pound