Fattening food, to a dieter

• Moral philosophy topic
• Something verboten
• Off-limits word
• Unacceptable practice
• Two votes against
• Pitching gem, in baseball slang
• Bit of naughtiness
• "Don't do that!"
• Trivial taboo

Wiktionary Word definitions in Wiktionary
n. (alternative spelling of no-no English)