"Impossible" originally was defined as something that is completely not possible to be done or completed. Modernly, it is a slang word, that is an adjetive. It is used for tasks that are of extreme difficulty, that one may have not completed. This one must have been the one to call it impossible, though in doubt, of his inability to do it, he knew it was not really impossible; just using it as an adjetive. Things that have not been completed by you, may be considerably impossible, due to your inability of the completion of what you are doing(trying).
Man 1: I'm going to run 5 miles in just 20 minutes.
20 minutes and 1 nanosecond later...
Man 1: I'm back- Oh, damn! Just 1 nanosecond off... this task is impossible!!!
Man 2: No, 'tis not impossible.
Man 2 runs 4.999999999 miles in just 19.99999999 minutes.
Man 1: You ran approximately 5 miles in approximately 20 seconds, see? ...it is impossible!!!!!
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