From Longman Dictionary of Contemporary Englishcardinal sinˌcardinal ˈsin noun [countable] 1 informalSTUPID/NOT SENSIBLE something bad or stupid that you must avoid doing politicians who commit the cardinal sin of ignoring public opinion2 RRCa serious sin in the Christian religion
Examples from the Corpus
cardinal sin• Also, eating is not likely to be viewed as a cardinal sin.• Monotony was a cardinal sin for Victorian architects, just as it is the predominant defining characteristic of modern architecture.• In Gandhi's view, its cardinal sin is the way it tolerates untouchability.• Senators who commit the cardinal sin of ignoring public opinion will soon be out of office.• It's fairly entertaining to watch but commits the cardinal sin of not identifying individual players.• I made the cardinal sin in tennis.• All things were possible; anyone could get rich; the cardinal sin was doubt. 