Archaic KJV Word
Let
Modern equivalent: restrain
What Was Lost
The theological weight of a divine restraining force actively holding back evil. Paul's passage describes someone powerfully letting (restraining) the mystery of iniquity -- a vivid image of cosmic spiritual warfare that reads as permission in modern English.
Closest Survivor in Modern English
without let or hindrance (survives in legal and tennis terminology)
Peak Usage (1611)
KJV 2 Thessalonians 2:7 -- 'He who now letteth will let, until he be taken out of the way'
Died ~1700
Complete meaning reversal -- the most dramatic in English. 'Let' shifted from 'hinder/restrain' to 'allow/permit,' making 2 Thessalonians 2:7 appear to say the opposite of its intended meaning.
What Replaced It
“hinder”
Implies mere obstruction; the old 'let' meant active, forceful restraint
“restrain”
Close but clinical; let-as-hinder carried a sense of holding back something powerful and dangerous
“prevent”
Ironically, 'prevent' took over the blocking meaning just as 'let' lost it -- a linguistic swap