Promissory estoppel

Estoppel is a collective name given to a group of legal doctrines in common law legal systems whereby a person is prevented from making assertions that are contradictory to his or her prior position on certain matters before the court - the person is said to be "estopped". Estoppel may operate by way of preventing someone from asserting a particular fact in court, or exercising a certain right, or from bringing a particular claim. Black's Law Dictionary defines estoppel as a "bar or impediment raised by the law, which precludes a man from alleging or from denying a certain fact or state of facts, in consequence of his previous allegation or denial or conduct or admission, or in consequence of a final adjudication of the matter in a court of law."

There are many different types of estoppel which can arise, but the common thread between them is that a person is restrained from asserting a particular position in law where it would be inequitable to do so. By way of illustration:

Estoppel is an equitable doctrine. Accordingly, any person wishing to assert an estoppel must normally come to the court with "clean hands".

The doctrine of estoppel (which may prevent a party from asserting a right) is often confused with the doctrine of waiver (which relates to relinquishing a right once it has arisen). It also substantially overlaps with, but is distinct from, the equitable doctrine of laches.

The verb is estop, which comes from Middle English estoppen, itself borrowed from Old French estop(p)er, estouper, presumably from Vulgar Latin *stuppāre ‘to stop up with tow, caulk’, from Latin stuppa, 'broken flax', from Ancient Greek stuppe, 'broken flax' The noun form estoppel is based on the Old French estoupail ‘stopper, bung’, a derivative of estouper.

Where a court finds that a party has done something warranting a form of estoppel, that party is said to be estopped from making certain related arguments or claiming certain related rights. The defendant is estopped from presenting the related defence, or the plaintiff is estopped from making the related argument against the defendant. Lord Coke stated, "It is called an estoppel or conclusion, because a man's own act or acceptance stoppeth or closeth up his mouth to allege or plead the truth."

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