Definition - What does Estoppel mean?
Estoppel refers to a principle that prevents or stops a person from going against or denying something in court that has already been established (as a truth) in a previous hearing. It also may be defined as adopting an attitude, position, or action which is against the fact established in court.
Justipedia explains Estoppel
For example, a person is prohibited from filing a lawsuit that raises an issue or includes a part, which has already been decided against them by another court.