special sauce

noun3

1. •
   A basic ingredient in McDonald's Big Mac hamburgers, a sort of Thousand Island dressing (now called Big Mac sauce by the company).

   • two all-beef patties, special sauce, lettuce, cheese, pickles, onions, on a sesame seed bun — 1975
2. •
   Burger sauce.
3. •
   (figurative) Synonym of secret sauce.