The word "decorous" describes good taste and proper etiquette. An example of a sentence using the word "decorous" is "The diplomats were decorous at all times during their visit to the embassy. "

(Decorous - characterized by dignified propriety in conduct, manners, appearance, or character)

Example: "Members of the diplomatic corps are expected to behave in a decorous manner."

Decorous is, essentially, dignified. An example sentence would be: He was feeling very decorous that day in his new suit.

It would translate to mean stiff behavior or manners. You can try saying

Use the word decorous just as you would use the word 'proper'

"Members of the diplomatic corps are expected to behave in a stiffly decorous manner."

Of sober or serious mien; composed and decorous in bearing; of modest look; staid; grave., Affectedly modest, decorous, or serious; making a show of gravity., To look demurely.