Related Clues:

• Forbidden act
• Not allowed
• Using metallic dishes in a microwave, e.g.
• Something not to talk about
• Cell phone ring in a theater, e.g.
• It's frowned upon
• Smacking one's lips while eating, e.g.
• Forbidden action
• Like a divorce, in some places
• Elbows on the table, e.g.
• Roving eyes on an exam, e.g.
• Don't
• Not permitted
• "___ Nanette"
• Morally wrong