a big no-no

Translations
большое табу
Wearing jeans to a formal event is a big no-no.
Надевать джинсы на официальное мероприятие — это большое табу.
Definitions
A big no-no refers to something that is considered unacceptable, inappropriate, or taboo in a particular context or situation.
Words
a

article or possessive pronoun
один
big

adjective
большой
no

noun
табу
no

noun
табу