Possible Clues:

• Something totally unacceptable (informal)
• Stealing someone's song, e.g
• Thing never to do
• Raiding the cookie jar, e.g
• Dessert, to dieters
• Modern composer's rules of grammar: double negatives are a ___ ?
• Taboo activity
• Transgression, to a child
• ''Don't do that!''
• Something you mustn't do