1. A reference to conventions or requirements of polite behavior: Jacob has always been considered a polite and decorous neighbor until he was heard yelling curses and vulgar words at his wife one evening.
2. Proper; becoming; suitable; with dignity and propriety: Even with indecorous customers, the clerk was respectable, polite, and the most decorous person in the store.

When people don't have wild parties that keep the neighborhood awake until dawn, are civil and courteous to strangers, and use language that is free of obscenities and blasphemies; then they are considered to be decorous, are living "decorously", and are not guilty of "indecorous" behavior.