Tin foil is a little hat people wear who are deeply into conspiracy theories or trying to pick up radio signals from aliens. Usually made of aluminum foil, also used to wrap food sometimes, or put on your TV antenna to get better reception.
foil is what police do to burglars, catching them in the act before they can get away with the loot. Foiled again, as they lead him off to jail. Or what the roadrunner does to Wiley Coyote, foil his plans every time.
Or, for the math "foil" see Karla S.'s answer. It's not as interesting though. F=First, O=outside, I= Inside, L=Last, for each term in a product of 2 binomials. Kind of boring. But First comes First, Last comes Last, as they live up to their names.