Before dismissing this conjecture, I have taken the pains to examine every one of the twenty-four other passages in which Milton has used the word "proof." I find that it occurs only four times as an adjective in all of which it is followed by something dependent upon it. In three of than thus:
"—— not proof
Against temptation."—Par. L. ix. 298.
"—— proof 'gainst all assaults."—Ib. x. 88.
"Proof against all temptation."—Par. R. iv. 533.
In the fourth, which is a little different, thus:
"—— left some part
Not proof enough such object to sustain."
Par. L. viii. 5S5.