I'm going to say that it's something they put on a big mac.....
I found this from wikipedia :
In 2012, McDonald's admitted that "the special sauce ingredients were not really a secret" because the recipe had been available online "for years". It consists of store-bought mayonnaise, sweet pickle relish and yellow mustard whisked together with vinegar, garlic powder, onion powder and paprika.
I remember my late grandma buying a sauce that you can get almost anywhere and it's very similar to the Big Mac sauce..but i don't know how healthy it is i know she used on her sandwiches and she lived to 85 so who knows LOL