1
general::
The honest and upright way of living, as in He led a wild life when he was young, but he's been on the straight and narrow for some years. This expression is widely thought to come from confusion of straight, "not crooked," with strait, "narrow," owing to a misinterpretation of a passage from the New Testament: "Strait is the gate, and narrow is the way, which leadeth unto life" (Matthew 7:14). The current phrase dates only from the first half of the 1800s.
American Heritage Idioms