Someone on the “straight and narrow” is living a legal, moral, and disciplined life.
The expression was referred to in The Pilgrim’s Progress by English writer John Bunyan (1628-1688).
In that inspirational book, Pilgrim, the representative of everyman, must follow the “straight and narrow.”
The phrase has a biblical origin in Matthew 7:14:
“Broad is the way that is the path of destruction but narrow is the gate and straight is the way that leadeth to the house of God.”