Origins
The verb 'let' is one of the most fundamental words in English, expressing the concept of permission — and yet its etymology reveals that 'letting' is not about actively granting anything but about stepping back, releasing control, allowing things to proceed without interference. This passive, non-interventionist concept of permission is encoded in the word's deepest roots.
Old English 'lǣtan' was a Class VII strong verb, conjugating lǣtan/lēt/lēton/lǣten (the modern forms let/let/let represent a radical simplification of this pattern). Its meaning was broad: 'to allow, permit, cause to, let go, release, leave behind, leave undone, bequeath.' The range from 'allow' to 'leave behind' to 'bequeath' makes sense under a unified concept: in all cases, the subject releases something from their control. You 'let' someone go by releasing your hold; you 'let' something happen by not preventing it; you 'let' (bequeath) property by leaving it behind for others.
Proto-Germanic *lētaną (to let, leave, allow) is reconstructed from cognates in all Germanic branches: Old Norse 'láta' (to let, allow, behave, cause), Gothic 'lētan' (to let, allow, leave), Old High German 'lāzzan' (modern German 'lassen,' to let, leave, allow), Old Saxon 'lātan,' and Old Frisian 'lēta.' German 'lassen' preserves the full range of Old English 'lǣtan' more faithfully than modern English 'let': German 'lassen' means both 'to let/allow' and 'to leave/abandon' and serves as an auxiliary verb in causative constructions ('ich lasse es machen' — I have it done, literally 'I let it be made').
Proto-Indo-European Roots
The PIE root *leh₁d- meant 'to let, leave, be weary, be slack.' The connection between 'letting' and 'being slack' is the conceptual key: to let is to slacken one's grip, to relax one's control, to become weary of resisting. The same root produced the adjective 'late' (from Old English 'læt,' slow, sluggish — originally 'slack, weary'), 'latter' (the later of two, from the comparative), and 'last' (from the superlative of 'late'). Latin 'lassus' (weary, tired — source of English 'lassitude') and Greek 'lēdein' (to be weary) are cognates from outside Germanic, confirming the PIE connection between releasing, slackness, and fatigue.
The homographic antonym 'let' meaning 'to hinder, obstruct' is one of English's most remarkable lexical oddities. This second 'let' comes from an entirely different Old English verb, 'lettan' (to hinder, delay, obstruct, make late), from Proto-Germanic *latjaną (to delay, hinder), from the adjective *lataz (slow, late). While the two verbs 'lǣtan' (to allow) and 'lettan' (to hinder) are ultimately related — both connect to the concept of slowness and delay — they had opposite meanings by the time of Old English. The 'hinder' sense has almost completely disappeared from modern English, surviving only in the tennis term 'let' (a serve that must be replayed because it was hindered by the net) and in the legal phrase 'without let or hindrance.'
In modern English, 'let' functions both as a full verb and as an auxiliary. As a full verb, it means 'to allow' (let me in, let the dog out, let it be). As an auxiliary, it forms first-person and third-person imperative or hortatory constructions: 'let us go' (hortatory — let's go), 'let him try' (jussive — he should be allowed to try), 'let there be light' (a creative fiat). The auxiliary use gives 'let' a unique grammatical function in English, one that has no exact parallel in most other Germanic languages.
Development
The contraction 'let's' (let us) has evolved so far from its full form that many speakers no longer parse it as 'let us.' Constructions like 'let's you and I go' or 'let's don't' treat 'let's' as an invariable particle rather than a verb with a pronoun object. This grammaticalization — the process by which a full verb becomes a functional element — is ongoing and illustrates how high-frequency verbs can erode into grammatical markers.
The property sense of 'let' (to let a house, to let rooms) preserves a Middle English extension of the 'allow/leave' meaning: to let a property is to allow someone to use it, to leave it in their hands. British English uses 'to let' (equivalent to American English 'for rent') as the standard real estate term. The noun 'sublet' (to let to a third party what one has already let from the owner) adds a layer of transference.
The phrasal verbs formed with 'let' are numerous and important. 'Let down' (to disappoint) imagines expectations as something held up that is then released to fall. 'Let in' (to admit) releases the barrier preventing entry. 'Let on' (to reveal a secret) releases concealed information. 'Let off' (to excuse from punishment, or to discharge a firearm) releases someone from the grip of justice, or releases the stored energy of an explosive. 'Let out' (to release, to make a garment wider) releases something from confinement. 'Let up' (to diminish, to relent) releases the pressure of sustained force. Each phrasal verb preserves the core 'release' meaning while specifying the direction or domain of release.
Later History
The phrase 'live and let live' (tolerance for others' choices) captures the deepest philosophy encoded in the verb: to let is to refrain from interference, to accept that others may live differently. 'Let it be' — the Beatles song title that has become a quasi-proverbial expression — distills the same philosophy into an imperative: release your need to control, allow things to exist as they are. Both phrases reveal that 'let' is fundamentally a word about the discipline of non-action, the courage of releasing control.
In programming, 'let' has been adopted in several modern languages (JavaScript, Swift, Rust) as a keyword for declaring variables, drawing on the mathematical use of 'let' to introduce assumptions ('let x = 5'). This technical use preserves the permissive sense: the programmer 'lets' a variable exist with a particular value. The mathematical tradition goes back to Latin translations of Arabic mathematical texts, where the equivalent of 'let' was used to set up hypothetical conditions.