ˈno-no BrE AmE noun [singular] informal
something that you must not do because it is considered to be unacceptable behaviour:
Colouring your hair was a no-no at that time.
ˈno-no BrE AmE noun [singular] informal
something that you must not do because it is considered to be unacceptable behaviour:
Colouring your hair was a no-no at that time.
Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012