Related Clues:

• It's just not done
• Second helping, to a dieter
• 'What were you thinking?!'
• Forbidden by social custom
• A profane state
• Taboo, to a child
• Never to be done
• Social impropriety
• Something you might get your hand slapped for doing
• Bad thing
• "Mustn't do" thing
• Talking with one's mouth full, e.g
• Texting while driving, e.g
• 4, something to be avoided
• Thing best avoided