cardinal sin 
noun
1(In Christian tradition) any of the seven deadly sins.The first complete medieval allegory, it depicts a world in which we are constantly at war with our own sinfulness, a struggle of faith between the cardinal virtues and the cardinal sins....
- After all who has never experienced the seven cardinal sins of Greed, Sloth, Wrath, Avarice, Envy, Pride or Lust before?
- Sixteenth century penitential books, which played an enormous part in popularising the concept of the cardinal sins through sermons and penance, continued to list eight sins rather than seven.
1.1chiefly humorous A serious error of judgement: he committed the cardinal sin of criticizing his teammates...
- I'd like to beg your collective indulgences while I commit the cardinal sin of bringing too much of the reviewer into the review.
- John Lauber, formerly an NTSB official and now an aviation safety counselor, once identified the seven cardinal sins leading to accidents.
- Too much of it is loose, self-indulgent and, the cardinal sin of political comedy, badly researched.