Something that is decorous is dignified, proper, and in good taste, like your decorous great-aunt who always wears a dress — even when she's only headed to the grocery store.
The adjective decorous shares its origins with decoration. Both words come from the Latin word decor, meaning "beauty, elegance, charm, grace, ornament." So, something decorous is attractive. Decorous can also mean "dignified or proper," like your decorous habit of saying, "How do you do?" when you meet someone for the first time.