What it means
Estoppel prevents a person from asserting something contrary to what they previously said or did, where another party reasonably relied on that conduct and would suffer harm if the position changed. In Australia, equitable and promissory estoppel can make an informal promise enforceable even without a formal contract, if it would be unconscionable to allow the promisor to break it. It is a flexible doctrine grounded in fairness and preventing detriment from broken assurances.
How it's used
Because he'd assured them he wouldn't enforce the deadline and they relied on that, estoppel prevented him from suddenly insisting on it.