"Secret Sauce" is a name applied to any condiment whose ingredients are unknown. There are many excellent and valid reasons for this:
- The owner of the restaurant would prefer that you didn't know.
- You would prefer that you didn't know (a la Tyler Durden's cream of mushroom soup).
- The minimum wage burger flipper in the back doesn't remember what he put in it.
Both the server and the diner have an unspoken rule that these reasons are ignored in favor of the charming and polite fiction that if the ingredients were made public, the restauranteur would be run out of business by others who made cheap knock-offs of his or her secret sauce.
Excellent examples include: all mayonnaise-based sauces at church suppers or pot lucks, anything on a Big Mac, the unlabelled bin of goo next to the Moo Goo Gai Pan at the Chinese Buffet, and anything on top of any meat product in any school cafeteria in America.
Also used as slang for semen (see Reason #2); also used as slang for any deep dark secret which must be protected. The secret of KFC's 7 herbs and spices is a parallel concept.