Estoppel is a judicial device in common law legal systems whereby a court may prevent or "estop" a person from making assertions or from going back on his or her word; the person being sanctioned is "estopped". Estoppel may prevent someone from bringing a particular claim. ... It is also a concept in international law.

When can estoppel be used?

Estoppel by acquiescence may arise when one person gives a legal warning to another based on some clearly asserted facts or legal principle, and the other does not respond within "a reasonable period of time". By acquiescing, the other person is generally considered to have lost the legal right to assert the contrary.

Can estoppel be used as a sword?

An estoppel by convention can arise where “the parties to a contract act on an assumed state of facts or law”. ... Estoppel by convention is to be used as a shield and not as a sword. The estoppel can come to end and will not apply to future dealings, once the common assumption is revealed to be erroneous.