secret sauce

noun5

1. •
   A sauce used in cooking or as a condiment, the ingredients of which are kept secret.

   • It was reported that the recipes for the secret sauce and grinder sandwiches were proprietary, known only to the current president of the corporation and the former owner of the restaurant. — 1996
2. •
   (informal, figurative) A secret idea or plan; a crucial element of something that makes it unique or functional.

   • The secret sauce to deliver connected systems within cities needs to contain the following ingredients. […]. — 2026
3. •
   burger sauce
4. •
   Synonym of special sauce.
5. •
   (humorous) Clandestine alcohol (alcoholic beverage), especially as a secret ingredient in a recipe or in a hip flask for surreptitious tippling.