So said Napoléon! In a phrase that is still in use today.
It conveys a feeling that all obstacles can be overcome and the battle can be won against all the odds.
Impossible n'est pas français was even the slogan for Eric Zemmour in the 2022 Presidential Election.
When I was told impossible n'est pas français, in my early days in Lyon, I took it quite literally for a while or as they say au premier degré. Eventually, I realised the true meaning!
In actual fact the word "impossible" comes from French and was formed into English in about 1410. Previously from the latin "impossibilis", which sounds to me like something out of a Monty Python sketch!
Just a note on the pronunciation of this word. The "IM" is pronounced just like "IN". There is no "MM" sound.