Archaic KJV Word
Simple
Modern equivalent: naive
What Was Lost
Proverbs' urgent pastoral concern. The simple ones were not stupid but unformed -- people who had not yet acquired wisdom and were therefore vulnerable to every deceiver. Proverbs pleads with the simple to gain wisdom before their openness destroys them. When simple became merely 'easy,' this urgent concern for the morally unprotected disappeared.
Closest Survivor in Modern English
simpleton (preserves the 'lacking discernment' meaning but as an insult)
Peak Usage (1611)
KJV Proverbs 1:22 -- 'How long, ye simple ones, will ye love simplicity?'; Romans 16:18 -- 'Deceive the hearts of the simple'
Died ~1900 (the 'morally naive/gullible' meaning faded)
Shifted from 'naive/easily deceived/morally unformed' (Latin simplex 'single-fold, without guile') to 'uncomplicated/easy,' losing the moral vulnerability dimension.
What Replaced It
“naive”
French import; simple-as-naive meant dangerously unprotected by wisdom, not charmingly innocent
“gullible”
Mocking; simple was pitying -- these were people who needed wisdom's protection
“easy”
The modern primary meaning -- no moral content whatsoever