From BiblePortal Wikipedia

King James Dictionary [1]

LET, pret. and pp. let. Letted is obsolete. To let out, like L. elocare, is to lease.

1. To permit to allow to suffer to give leave or power by a positive act, or negatively, to withhold restraint not to prevent. A leaky ship lets water enter into the hold. Let is followed by the infinitive without the sign to.

Pharaoh said, I will let you go.  Exodus 8 .

When the ship was caught and could not bear up into the wind, we let her drive.  Acts 27 .

2. To lease to grant possession and use for a compensation as, to let to farm to let an estate for a year to let a room to lodgers often followed by out, as, to let out a farm but the use of out is unnecessary. 3. To suffer to permit with the usual sign of the infinitive.

There's a letter for you, Sir, if your name be Horatio, as I am let to know it is. Not used.

4. In the imperative mode, let has the following uses. Followed by the first and third persons, it expresses desire or wish hence it is used in prayer and entreaty to superiors, and to those who have us in their power as, let me not wander from thy commandments.  Psalms 119 .

Followed by the first person plural, let expresses exhortation or entreaty as, rise, let us go.

Followed by the third person, it implies permission or command addressed to an inferior. Let him go, let them remain, are commands addressed to the second person. Let thou, or let ye, that is, do thou or you permit him to go.

Sometimes let is used to express a command or injunction to a third person. When the signal is given to engage, let every man do his duty.

When applied to things not rational, it implies allowance or concession.

O'er golden sands let rich Pactolus flow.

5. To retard to hinder to impede to interpose obstructions.  2 Thessalonians 2 .

This sense is now obsolete, or nearly so.

To let alone, to leave to suffer to remain without intermeddling as, let alone this idle project let me alone.

To let down, to permit to sink or fall to lower.

She let them down by a cord through the window.  Joshua 2 .

To let loose, to free from restraint to permit to wander at large.

To let in or into, to permit or suffer to enter to admit. Open the door, let in my friend. We are not let into the secrets of the cabinet.

To let blood, to open a vein and suffer the blood to flow out.

To let out, to suffer to escape also, to lease or let to hire.

To let off, to discharge, to let fly, as an arrow or cause to explode, as a gun.

LET, To forbear. Obs.

LET, n. A retarding hinderance obstacle impediment delay. Obsolete, unless in some technical phrases.

LET, a termination of diminutives as hamlet, a little house rivulet, a small stream. See Little.

Webster's Dictionary [2]

(1): ( n.) A stroke in which a ball touches the top of the net in passing over.

(2): ( v. t.) To retard; to hinder; to impede; to oppose.

(3): ( v. t.) To cause; to make; - used with the infinitive in the active form but in the passive sense; as, let make, i. e., cause to be made; let bring, i. e., cause to be brought.

(4): ( n.) A retarding; hindrance; obstacle; impediment; delay; - common in the phrase without let or hindrance, but elsewhere archaic.

(5): ( v. t.) To leave; to relinquish; to abandon.

(6): ( v. t.) To consider; to think; to esteem.

(7): ( v. i.) To forbear.

(8): ( v. i.) To be let or leased; as, the farm lets for $500 a year. See note under Let, v. t.

(9): ( imp. & p. p.) of Let

(10): ( v. t.) To permit; to allow; to suffer; - either affirmatively, by positive act, or negatively, by neglecting to restrain or prevent.

(11): ( v. t.) To allow to be used or occupied for a compensation; to lease; to rent; to hire out; - often with out; as, to let a farm; to let a house; to let out horses.

(12): ( v. t.) To give, grant, or assign, as a work, privilege, or contract; - often with out; as, to let the building of a bridge; to let out the lathing and the plastering.

Hastings' Dictionary of the Bible [3]

LET. In Anglo-Saxon lætan meant ‘to permit’ and lettan , ‘to hinder.’ In course of time both words were spelled ‘let.’ Consequently in AV [Note: Authorized Version.] , besides its modern meaning of ‘permit,’ the vb. ‘let’ sometimes has the opposite meaning of ‘hinder.’ Thus   2 Thessalonians 2:7 , ‘only he who now letteth will let, until he be taken out of the way.’ The other places are   Exodus 5:4 ,   Numbers 22:16 m,   Isaiah 43:13 , Wis 7:22 ,   Romans 1:13 .

People's Dictionary of the Bible [4]

Let.  Exodus 5:4, R. V., "loose;"  Isaiah 43:13;  Romans 1:13, R. V., "hindered;"  2 Thessalonians 2:7, R. V., "restraineth." This word is used in old English for "to hinder," "to stop."

American Tract Society Bible Dictionary [5]

Sometimes used in the Bible in the old English sense, that is, to hinder,  Isaiah 43:13;  Romans 1:13 .

Vine's Expository Dictionary of NT Words [6]

 Romans 1:13  2—Thessalonians 2:7

International Standard Bible Encyclopedia [7]

( κατέχω , katéchō ): Usually in the sense of "permit" (Anglo-Saxon, leatan ), but also in Old English with meaning of "hinder" (Anglo-Saxon, lettan ). This latter sense is found in   2 Thessalonians 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."