Meaning of Impossible  

After two years of working together, I finally knew for sure what I had long suspected: “Stupid” was just an expression Feynman applied to everyone, including himself, as a way to focus attention on an error so it was never made again.

I also learned that “impossible,” when used by Feynman, did not necessarily mean “unachievable” or “ridiculous.” Sometimes it meant, “Wow! Here is something amazing that contradicts what we would normally expect to be true. This is worth understanding!”

Prof. Paul J. Steinhardt on What Impossible Meant to Feynman.