The story
Attested since c. 1962, originally Southern United States English; popularized by T. Bert Lance in May 1977.
Etymology from Wiktionary, CC BY-SA 4.0.
if it ain't broke, don't fix it
Today it means: It is wise to leave something alone and to avoid attempting to correct, fix, or improve what is already sufficient (often with an implication that is the attempted improvement is risky and might backfire). Full definition →