Transgression, to a child

• Taboo thing: Hyph
• Eating with one's elbows on the table, e.g
• Item on a censor's hit list
• Forbidden behavior
• Like Eden's apple
• Improper act
• Thing a toddler shouldn't do: Hyph
• Something best avoided
• It's to be avoided

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