Definitions for no-no
Random House Webster's College Dictionary
Informal.anything that is forbidden or not advisable, as because of being improper or unsafe.
Category: Common Vocabulary, Informal
Origin of no-no:
Kernerman English Learner's Dictionary
sth that is completely unacceptable
Criticizing your friends' family is a big no-no.
Something that is forbidden, prohibited, discouraged or taboo.
Smoking indoors is becoming a real no-no these days, in many places.
A no-hitter; a game in which no batter on one of the teams got a hit.
He threw two no-nos in his career.