The personal quality of decency is one of honesty, good manners, and respect for other people.
Over time, decency has referred to manners, but today decency is mainly a strong sense of right and wrong, and a high standard of honesty. When a criminal or dictator does horrible things, people assume they have no sense of decency. When a tasteless or violent TV show becomes popular, some people wonder if society has lost its sense of decency. Asking "Have you no decency?" is a serious question, unless you're being silly and complaining about something like homework.