Left/Right, I believe, came from the Assembly after the French revolution. The parties split into factions that sat on the left and the right side of the hall, and the left were the more radical side. Later, when socialism rose, they adopted that terminology. It doesn’t really apply to America, we are not in a spectrum between pure communism and pure whatever its opposite is, we are a free market republic with supposedly limited government, and our spectrum is on an entirely different line from theirs.
Left is sinister and Right is justified.