[something] is a major no-no

Translations
[что-то] — это большое табу
In this office, being late is a major no-no.
В этом офисе опоздание — это большое табу.
Definitions
The phrase '[something] is a major no-no' means that the specified action or behavior is strongly discouraged, unacceptable, or forbidden in a particular context.
Words
no-no

noun
табу
major

adjective
основной