Bible Dictionary
LET
In older English, let may mean either to permit or to hinder. In 2 Thessalonians 2:7 it has the older meaning “restrain,” referring to what holds back the lawless one.
Biblical Usage
In 2 Thessalonians 2:7 the King James Version uses “letteth” and “will let” for the one who restrains the lawless power. The Revised Version renders the sense “there is one that restraineth now.”