A more thorough explanation:

An assignor is a person who transfers property rights or powers to another. They are also known as assigners. For example, if someone sells their car to another person, they are the assignor of the car's ownership rights.

Assignor estoppel is a legal principle that prevents an assignor from later claiming that the property they assigned did not belong to them. This means that if the assignor sells a car to someone and later tries to claim that they did not have the right to sell it, they cannot do so because they already transferred the ownership rights to the buyer.

Another example of an assignor is in the context of a lease. If a tenant sublets their apartment to another person, they are the assignor of the lease agreement to the subtenant.