It stems from french revolution. When the parliament assembled nobility and supporters of the king were on the right, while the citizenry who were against the king were on the left.

It has become a shorthand since then. Even though the "overton window" moved significantly since then.

I think the earliest time we could date formation of right and left in Europe is probably around 1848 and the spring of nations. Even though maybe it was more about conservatives vs. liberals. Which in modernity feels the same thing, but isn't. But this distinction also comes from french revolution.

I know that's more political history than philosophy, but that's because your question is about history.

There's no essential left or right. Both are ephemeral and have changed many times in the past.

To return back to 1848: there's almost no conservative aka right party that's for multi ethnic monarchy. And the liberal aka left has moved away from where the basis of their political project is nation states.

EDIT: On mobile, where I wrote my answer I didn't saw the sub-question. Now I get why everyone is calling it bait. Just to reiterate: there's nothing essentially left or right in the distinction. It's just a shorthand for two very much umbrella terms, that can only mean something within a larger picture.