Related Clues:

• Routine
• Equestrian's garb
• Emphatic denial
• Transgression
• Taboo
• Banned act
• Faux pas
• It's outlawed
• 'That's a ___!'
• Talking during a film, e.g.
• Thou-shalt-not
• Forbidden fruit
• Proscription
• Equestrian's attire
• Double negative?