lol, this is brilliant.
Of course "rent free" is something I've read somewhere. All the words we are communicating with I also read somewhere. That's how language works!
It's a figure of speech, albeit a more modern one, maybe that's why it so obviously irks you.
I notice you used the old "scoring out of 10" thing. I've seen that one used loads before!
Again, because that's how language works. Go ahead and be original, invent some new words or expressions if you like, stop being so unoriginal.
My favourite part is that you couldn't help yourself but bite and show me that the expression winds you up too. I was just using the expression as a tool of communication, this is an unexpected bonus!