Wearing jeans to the formal dinner was considered an impropriety.

impropriety 是由前缀 im-（表示"不、非"）和 propriety（"得体、恰当"）组成的名词。propriety 来自 proper（"适当的"），所以 impropriety 字面意思是"不得体、不恰当"，指违反社会礼仪或规范的行为，即"失礼"。