A more thorough explanation:

Estoppel in pais, also known as equitable estoppel, is a legal defense that prevents a person from using a right against another person when that right arises from misleading actions. This doctrine was created to prevent someone from taking unfair advantage of another person through legal means.

For example, if someone leads their partner to believe that they are legally married, they cannot later use the illegitimacy of the marriage as an argument against splitting property in a divorce proceeding. This is because the partner relied on the misleading conduct of the other person and suffered harm as a result.

The requirements for estoppel in pais vary by state, but generally, the party claiming the right must have knowingly misled the other party, the other party must have relied on that conduct, and the other party must have suffered some harm. Some states require a higher standard of willful conduct, while others allow negligence to be sufficient for misleading someone. Additionally, the person using the estoppel defense must have reasonably relied on the misleading conduct of the other party.

Overall, estoppel in pais is a legal defense that prevents someone from using a right that arises from misleading conduct. This doctrine is meant to protect individuals from being taken advantage of through legal means.