Taking this cookie is a no-no.
The definition of a no no is something that should not be done or something that is against the rules.
An example of a no no is eating cookies before dinner after your mom said not to eat them.
Slang something that is forbidden or considered unwise to do, say, use, etc.
Origin of no-no; from No! No!, as said in admonishing a young child
nounpl. no-noes Informal
- Something unacceptable or impermissible: “Even though his company wasn't the one involved in the case, what he did is considered a definite no-no” (Mike Royko).
- A social blunder; a faux pas.
- Baseball A game in which a pitcher allows no hits and no runs.
- Alternative spelling of no-no.