How do we know the meaning of Arabic words hasn't changed?

I somewhat disagree with the answer given ln by @goldpseudo.

The Arabic language - restricted on the words of the qur'an- has not changed. But similarly as the restriction made by 'Othman ibn' Affan in his compilation of the qur'an by favoring the language of Quraish. The - in use- language has restricted certain meanings. And mainly killed the diversity of the language. Nevertheless in some Arabic dialects older meanings still exist and are applied until today. But people who are not familiar with these meanings and words may consider these dialects as strange. This is due to the fact that Arabic speakers now in practice don't speak or use the standard Arabic or the Arabic of the qur'an anymore in most if not all their daily life.

This is the main reason why many native Arabic speakers may have issues understanding the qur'an. As the language they practice is far away from it if may only know one meaning out of many other different meanings. For example the word:

الحوت

In classical Arabic doesn't refer only to whale, but to fish in general too. The later meaning is only used in certain dialects of North Africa by now while all other Arabs laugh when they hear people inviting them to eat الحوت and then they only find what they know as:

السمك

You see, the problem here is that the Arabic language has changed. Which can be said for pretty much any other language that has been in use for over fourteen centuries. Arabic, as it is commonly spoken and understood around the world, really has little in common with the Arabic spoken during the time of the Prophet and encoded in the Qur'an.

Heck, even the Arabic language at the time had multiple dialects and the Qur'an itself has multiple variant readings which are all considered canon.

As Muslims, where we put our ultimate faith is important, and it is common especially in such interfaith debates for ones faith to end up misplaced: Are we putting our faith in language, the vagaries of which can change over hundreds of years? Are we putting our faith in ink and paper, which is fragile and recorded by imperfect men? Or are we putting our faith in Allah Al-Hadi Ar-Rashid, who can protect His message and guide us to the right path no matter what?