To prevent or obstruct an anticipated event or action by taking advance action. The company took measures to forestall any potential financial losses by diversifying its investments.
To act in advance of someone in order to prevent them from doing something. She quickly spoke up to forestall any objections from the committee.
Archaic: To intercept and buy up (goods) before they reach the market, so as to resell at a higher price. Medieval laws punished those who tried to forestall grain on the road to the city.