Related Clues:

• It's prohibited
• Sin, to tots
• Something banned
• Something off-limits
• Off-limits word
• Prohibited act
• Don't do it
• It's strictly taboo
• Taboo thing
• Rich dessert, to a dieter
• Putting regular gas in a diesel engine, e.g.
• It's to be avoided
• It's taboo
• It's a bad thing
• Admonishing utterance