From Longman Dictionary of Contemporary Englishlet the side downlet the side downBritish EnglishDISAPPOINTED to behave badly or do something that embarrasses or disappoints your family, friends etc → side
Examples from the Corpus
let the side down• Brown was constantly letting the side down.• It is an unmentionable subject, a terrible way of letting the side down.• Essentially, it's the ageing drivetrain that lets the side down.• I don't want to let the side down - don't send me to the Sick Room! 