Free: to have choice and be unfettered
Thinker: someone who is able to examine options in a logical and reasoned way, (but literally, not necessarily, just to have thought.)
Together: in close proximity or as a unit

I think "Free thinkers" evolved as a group name because when they first got started (over a century ago) it was "taboo" to be "anti-religious"...so instead of "coming out" and saying they were "agnostic" or "atheistic" or even worse..."anti-religious" (I love Bill Maher's term...#Religulous) they chose to identify with "free" (as in unrestricted) "thinker" as opposed to being "Christian" or "Muslim".

You'll like what was written here;

[en.wikipedia.org]

. . . hmmm, free :

For me, free is the choice to give myself the greatest latitude of thought and action that doesn't impinge on another person's latitude.
Thinker is to figure things out for myself using what I've learned from my own experience and the wisdom of others.
Free thinker is having the ability to do this.

Freedom is a noble word

Freedom is a noble word
When used poetically
Conveying that the soul of man
Constrained will never be.

Freedom has another sense
Elusive to translate
Where freedom means its opposite
As now I hereby state.

When you buy a camera
Which brags its "Focus Free"
It means it's cheap and basic and
Good shots you'll seldom see.

You spot some super software
It says the download's free
But, later, it says "activate"
And then there is a fee.

A "free lunch" means your supper
May have a cost you rue.
For in return for luncheon snack
The dinner bill's on you.

Yes. Freedom has a double edge
And when you're truly free
There's no one looking after you.
You trade security.

If you think I'm cynical;
My words belike you not,
Remember too that freedom means
You're free to call them rot!

© Petter Finne

Yes, what Dave said.... ☺

The word free means...unhindered and allowed to follow our own instincts or beliefs. The word thinker means someone who uses critical thought and logic to arrive at conclusions independent of the influence of others. Together the word freethinker describes someone who freely and without bias, forms their opinions and beliefs based purely on evidence and logic.