secret sauce

Short definition from Wiktionary

Noun

1. 1
   A sauce used in cooking or as a condiment, the ingredients of which are kept secret.
2. 2
   A sauce used in cooking or as a condiment, the ingredients of which are kept secret: Synonym of special sauce.
3. 3
   Synonym of special sauce: burger sauce
4. 4
   A secret idea or plan; a crucial element of something that makes it unique or functional.
5. 5
   Clandestine alcohol (alcoholic beverage), especially as a secret ingredient in a recipe or in a hip flask for surreptitious tippling.