Words ending in -or and -ee (such as promisor/promisee) are commonly found in legal texts of all kinds, but particularly in contracts. In these words, the -or indicates the person initiating the action, and the -ee ending the one receiving it. Thus promisor refers to a person making a promise, while the promisee is the recipient of the promise, or the person to whom something has been promised. Note that words of this type are also found in everyday English (for example employer, someone giving employment; employee, someone receiving employment).
Amy Krois-Linder, International Legal English, Professional English, Cambridge.