the.com/straight and narrow

means The morally upright, rule-following path of behavior — doing what's right and proper rather than what's easy or tempting.

from Lifted from the Bible, Matthew 7:14, which in the King James translation speaks of the 'strait' gate and 'narrow' way that lead to life, contrasted with the wide, easy road to destruction. 'Strait' meant tight or constricted (as in straitjacket, straits) — not 'straight' at all. Over centuries, as 'strait' faded from everyday vocabulary, popular usage swapped it for the more familiar 'straight,' reshaping the phrase into 'straight and narrow.' So the idiom is really a 400-year-old spelling drift: a path once described as narrow and *tight* became narrow and *undeviating* in the public imagination — fitting, since the moral path it describes is supposed to have no room for wandering either way.