Where did the saying, "I'm not gonna lie" come from?

[hbarrass]

I believe you are thinking of George Washington who, according to legend confessed to killing his father's cherry tree.

From the legend by Parson Weems:

"George," said his father, "do you know who killed that beautiful little cherry tree yonder in the garden?"

This was a tough question; and George staggered under it for a moment: but quickly recovered himself: and looking at his father, with the sweet face of youth brightened with the inexpressible charm of all-conquering truth, he bravely cried out, "I can't tell a lie, Pa; you know I can't tell a lie. I did cut it with my hatchet."

"Run to my arms, you dearest boy," cried his father in transports, "run to my arms; glad am I, George, that you killed my tree; for you have paid me for it a thousand fold."