Related Clues:

• Off limits
• 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
• Cultural no-no
• Admonishing utterance
• Something best avoided
• Deed to avoid
• Something that's taboo
• Baby's taboo
• Tot's taboo
• Proscribed act
• Smoking while pumping gas, e.g.