Un ceremonious means informal or casual, but if you want to describe the opposite of this term, you can use antonyms such as formal, ceremonial, decorous, courteous, or respectful. A formal occasion requires appropriate attire and strict adherence to protocol, while an informal occasion allows for a more relaxed dress code and conversational tone. Ceremonial events are marked by pomp, circumstance, and solemnity, while unceremonious events are characterized by simplicity and ease. Decorous behaviour is characterized by good taste and propriety, while discourteous behaviour is marked by rudeness and disrespect. Respectful interactions are courteous and polite, while disrespectful interactions are impolite and offensive.