What is "live-in" relationship? Where is the culture Originated from? Which Country has highest number of them

Answer 1

"Live-in" usually refers to two people living together.
I don't know who first called it that, but since it's an English word, it started in some country where the people speak English.
As to where it is most common, that's hard to say, since some people don't want others to know their living arrangements, and might hide them.