To anticipate; to take beforehand. Why need a man forestall his date of grief, and run to meet what he would most avoid?

To hinder by preoccupation or prevention. I will not forestall your judgment of the rest.

In law, to buy or bargain for corn, or provisions of any kind, before they arrive at the market or fair, with intent to sell them at higher prices. This is a penal offense.

To deprive by something prior.