From Longman Dictionary of Contemporary Englishlet somebody/something ↔ down phrasal verb1 DISAPPOINTEDto not do something that someone trusts or expects you to do She had been let down badly in the past. The worst feeling is having let our fans down.let the side down British English (=disappoint a group of people that you belong to)2 to make someone or something less successful or effective McKenzie’s judgement rarely lets him down.3 DOWNto move something or someone to a lower position Let down a rope so that I can climb up. Carefully, she let herself down into the water.4 let your hair down informalRELAXED to relax and enjoy yourself, especially after working hard Visitors young and old let their hair down and enjoyed the show.5 let your guard/defences down to relax and stop worrying about what might happen or what someone might find out about you Maggie never really lets her guard down, does she? 6 let somebody down lightly/gentlyTELL to give someone bad news in a way that will not upset them too much I get asked out on dates quite often, but I always try to let the guy down gently.7 British EnglishAIR to allow the air to escape from something so that it loses its shape and becomes flat Someone’s let my tyres down!8 DCto make a piece of clothing longer by unfolding a folded edge OPP take upGRAMMAR: Comparisonlet• You let someone do something: Her boss lets her work from home. ✗Don’t say: let someone to do somethingLet is not used in the passive.allow• You allow someone to do something: Her boss allows her to work from home.• Something is allowed: Working from home is allowed.Allow is often used in the passive.permit• Something is permitted: Working from home is permitted.• Someone is permitted to do something: Employees are permitted to work from home.Permit is usually used in the passive. → let→ See Verb table
Examples from the Corpus
let the side down• Brown was constantly letting the side down.• Essentially, it's the ageing drivetrain that lets the side down.• It is an unmentionable subject, a terrible way of letting the side down.• I don't want to let the side down - don't send me to the Sick Room!let your hair down• He liked this: what his pub was all about, for people to let their hair down.• Man's got ta let his hair down.• Out in the pasture, the princess let her hair down.• This was the day our friends let their hair down and spoke with amazing frankness.• We know when we can afford to let our hair down and when we can't.• In the second half Complicite let their hair down in their own inimitable way.• Among the many booksellers and publishers whom I spotted letting their hair down on the dance floor was independent publisher Christopher Hurst.let your guard/defences down• We must not let our defences down, Mrs Thatcher and other cautious voices would argue.• Never let your guard down was the only solace he offered. 