no–no 
/ˈnoʊˌnoʊ/ noun
plural no–no's or no–nos /ˈnoʊˌnoʊ/ noun [count] informal 1 : something that people are not supposed to do because it is not proper, safe, fashionable, etc.
▪ Forgetting to introduce your guests to one another is a big no-no when hosting a party. ▪ Sharing prescription medication with other people is a definite no-no. ▪ fashion no-no's 2 baseball : no-hitter
▪ Forgetting to introduce your guests to one another is a big no-no when hosting a party. ▪ Sharing prescription medication with other people is a definite no-no. ▪ fashion no-no's
