Estoppel in pais (also called equitable estoppel) is a defense doctrine that prevents a party from using a right against another party when the right arises out of misleading actions from the person claiming the right. The doctrine arose under common law as a method for preventing someone from taking unfair advantage of another through legal channels. For example, if someone misleads their partner to believe they were legally married, they cannot use the illegitimacy of the marriage later in divorce proceedings as an argument against splitting property in half.
The requirements for estoppel in pais differ by state, but many states require that the party claiming the right knowingly misled the other party, the other party relied on that conduct, and the other party suffered some harm. Some states require stricter willful standards while others allow negligence to suffice for misleading someone. Also, states often have other requirements like the person using the estoppel defense must reasonably rely on the misleading conduct of the other party.
[Last updated in June of 2021 by the Wex Definitions Team]