No-no (noun)

Something that is not allowed or acceptable.

Origin:

Informal, shortened form of the phrase "no go" or "not to be done".

Examples:

1. Smoking is a big no-no in the office.
2. Bringing weapons to school is a no-no.
3. Cheating on an exam is a no-no.
4. Skipping class is a no-no for students.
5. Wearing flip-flops to a formal event is a no-no.