Why is the root word of impossible is possible?

Impossible things are generally regarded as impossible due to several key reasons:

1. Many have tried, many have failed (e.g. flying, not gliding, for an extended period of time without a motor or the like) This is inductive reasoning; all OBSERVED attempts have failed, so ALL will fail

2. It disobeys the laws of physics (e.g. rain falling up)

3. General acceptance/lack of technology or knowledge (e.g. in the 1800's, flying to the moon was "impossible")

I hope I was able to answer your question :)