The word decorum eomes from the Latin word decorus, meaning "proper" or "fitting", which itself is derived from the Latin decor, meaning "beauty, grace, or ornament".  It entered English in the 16th century, initially referring to what is "proper or fitting" in a literary or artistic composition before broadening to mean proper and polite behavior. 

According to the Oxford English Dictionary, the word "decorum" was first used in English in the mid-1500s, with the earliest evidence from before 1568 in the writings of Roger Ascham.  The word comes from the Latin word decōrum.  The term was first recorded in English around the 1560s to describe literary and artistic propriety, and by the 1580s, it was being used to mean "propriety of speech, behavior, or dress". 

Examples of "decorum" used in a sentence:  
"it was important to maintain decorum in the council chamber." 
"At the start of the hearing, the judge warned the audience to keep decorum in the courtroom." 
"There are some democracies where business is conducted with correctly soporific decorum." 
"They have a reputation for decency and decorum while playing a tough and skillful game." 
"Suddenly, all decorum and perspective was out the window." 
"The public should have high expectations of politicians in terms of their personal conduct and decorum." 
"In a place where manners and decorum are at the absolute bottom of the priority heap, all that is really required of you is to avert disaster."