Where did the saying come from I have forgotten more than you will ever know?

My father-in-law used to say it in the late '70s and he was around 70 at the time. I understood that he had heard it a long time before. I do not agree with some who think it is an insult tossed by an older person at a younger one just to be ornery. I believe it is used when a more experienced person of any age is told by another person of any age that their experience/knowledge is equal when it clearly isn't. In other Words, it is a RESPONSE to an insult to and lack of respect for an elder in some field. An example would be a surgical resident mouthing off to a surgeon and telling him how to perform an operation.