I think "source" is correct, and tasting it is supposed to be akin to drinking from the Fountain of Youth. While the Big Mac had already been introduced back in 1968, the "Special Sauce" etc. slogan didn't hit the air until 1975, and I don't recall hearing the expression "secret sauce" much until a while after that. "Secret recipe," yes, but the archetypical secret recipe was always "eleven herbs and spices," not a sauce.
I never did learn to say "bun seed sesame a on onions pickles cheese lettuce sauce special patties beef all two."
FWIW
Date: 2005-09-09 09:06 am (UTC)I never did learn to say "bun seed sesame a on onions pickles cheese lettuce sauce special patties beef all two."