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.
Definition
In older English, let could mean either to permit or to hinder. In the biblical phrase “he who now letteth,” it has the older meaning “he who restrains.”
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.”
Historical Background
The two meanings reflect different Old English verbs: one meaning to permit and another meaning to hinder.
Historical Bible Dictionaries
International Standard Bible Encyclopedia (ISBE)
<p>(katecho): Usually in the sense of "permit" (Anglo-Saxon, leetan), but also in Old English with meaning of "hinder" (Anglo-Saxon, lettan). This latter sense is found in 2Th 2:7 the King James Version, "Only he who now letteth will let," where the Revised Version (British and American) has, "Only there is one that restraineth now."</p>