prevent by taking action in advance
Even though the dog that bit you was probably not rabid, you should still take the antidote to forestall the remote possibility of infection.
stall is to stop. fore means before. i. e fore + stall = taking precautions before itself, to prevent unwanted action. Fore means before. Stall means to stop. Hence Forestall == Before Stop. i. e to stop something before it happens.