> The cornerstone of our strategic policy continues to be to deter nuclear attack upon the United States or its allies. We do this by maintaining a highly reliable ability to inflict unacceptable damage upon any single aggressor or combination of aggressors at any time during the course of a strategic nuclear exchange, even after absorbing a surprise first strike. This can be defined as our assured-destruction capability.
> Security depends upon assuming a worst plausible case, and having the ability to cope with it. In that eventuality we must be able to absorb the total weight of nuclear attack on our country -- on our retaliatory forces, on our command and control apparatus, on our industrial capacity, on our cities, and on our population -- and still be capable of damaging the aggressor to the point that his society would be simply no longer viable in twentieth-century terms. That is what deterrence of nuclear aggression means. It means the certainty of suicide to the aggressor, not merely to his military forces, but to his society as a whole.
https://www.atomicarchive.com/resources/documents/deterrence...
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McNamara used the term "assured destruction" over and over again it became a cornerstone of his policy. Later a civilian added an "M" to MAD, and the term caught on.
Here's an interview transcript from much later, in 1987:
INT: Much has been made during this period of the term 'MAD', Mutually Assured or..
RM: Yes..
INT: ......Destruction. Can you explain what was meant by that?
RM: It's not mad! (laugh) Mutual Assured Destruction is the foundation of deterrence.
RM: Today it's a derogative term , but those that denigrate it don't understand deterrence. If you want a stable nuclear world -- if that isn't an oxymoron -- , to rephrase it, to the degree one can achieve a stable nuclear world, it requires that each side be confident that it can deter the other.