The Greek word "prototokos"
"Firstborn"

Did anyone search out the true meanings of this word?

You can find the meaning here Crosswalk.com

Colossians 1:15 (New King James Version)
He is the image of the invisible God, the firstborn over all creation.

To many of us the word firstborn simply means the first to have been born. But if you look into it you’ll find that the scriptures also have another meaning for this word. Take a look at the following.

Genesis 41:51-52 (New King James Version)
Joseph called the name of the firstborn Manasseh: “For God has made me forget all my toil and all my father’s house.” And the name of the second he called Ephraim: “For God has caused me to be fruitful in the land of my affliction.”

Jeremiah 31:9 (New King James Version)
They shall come with weeping, And with supplications I will lead them.
I will cause them to walk by the rivers of waters, In a straight way in which they shall not stumble; For I am a Father to Israel, And Ephraim is My firstborn.

Notice that this Joseph called his firstborn Manasseh and that in the book of Jeremiah God calls his second son his firstborn. Thus we see that the word firstborn has more then one meaning.

Why am I bringing this up? Because some would have us believe that God’s Son was created when in fact the word firstborn in it’s proper context simply means that He has all supremacy. Paul's usage of the word firstborn (prototokos) in Colossians 1:15 simply refers to Christ's supremacy and not His birth.

Thus the word firstborn (Prototokos) in it’s proper context is a legal term and not a biological one. It simply means, "first in rank, status, pre-eminent, supreme, superior, and unique."

As you saw, a person can be firstborn but not born first.