"pretty please"

Would like to know the origin of this phrase? How can please be pretty?

Under the meaning of "pretty penny," it says: ".Pretty has several meanings beside 'pleasing to the eye.' It also means 'considerable'." From the Morris Dictionary of Word and Phrase Origins by William and Mary Morris (HarperCollins, New York, 1977, 1988). That would explain "pretty please." One is REALLY saying please, a whole big bunch, a considerable amount.