Estoppel. It seems like a funny word. But the equitable doctrine of a estoppel as a useful tool in a litigator’s tool box.
The rational for an estoppel is to "prevent a party from going back on his word (whether expressed or implied) when it would be unconscionable to do so."
An estoppel arises where:
When does an estoppel arise?
An estoppels most commonly occurs in a contractual situation where one party has a right but says or represents to another party that they will not exercise that right.
For example, assume parents own a farm and they want to ensure that their child takes over the farm and do so by a sale and purchase agreement with a debt back. If the parents promise the child that the debt would not be called up then an estoppel may arise. It may arise if the child relied upon that representation, it was expressed clearly and equivocally, the child acted to his or her detriment (say by forgoing other opportunities in life) and it would be unconscionable for the parents to go back on the promise not to call up the debt.
There are numerous other examples in the commercial context. An estoppel is a remedy in equity and enables the Court to look at the justice of the case and balance competing interests.
While not all estoppel cases go to Court to be determined it does provide a useful tool to encourage parties to settle a dispute.
 National Westminister Finance NZ Limited v National Bank of New Zealand Limited  1 NZLR 548 (CA) at .