To prevent or delay an anticipated event or action by taking advance measures. To act in advance to prevent something from happening.
Originally from Old English foresteal (an ambush), combining fore- (before) and steal (to place secretly). Medieval merchants would 'forestall' by intercepting goods before they reached market to control prices. The meaning evolved to any preemptive action that prevents something.
Think of 'forestall' as setting up a 'forest wall' to block something before it arrives! You're creating an obstacle in advance. It's like arriving early at a movie theater to forestall having to sit in the front row.
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