I don't know what you mean by" entered the public lexicon ", if you mean when the public was first aware of the term, or when the public started using the term for it's own uses.
I first became aware of it in 1962 during the Cuban missile crises, just 16 at the time, but with 2 family members in the military, I was watching things.
So a lot of civilians probably became aware of it at about the same time, and started using the term for their own purposes .
When I got out of the military in 73, I'd occasionally hear it used by civilians to describe things not going so smoothly at the work place.
But they usually have no idea what 1-5 meant, and which number meant better or worse.