His undutiful behavior towards his parents disappointed the whole family.

undutiful 是由前缀 un-（表示"不、非"）和形容词 dutiful（尽职的、恭敬的）组成的复合词。dutiful 来自名词 duty（责任、义务），加上后缀 -ful 构成形容词，表示"尽责的"。 undutiful 字面意思是"不尽责的"，引申为"不恭敬的、无礼的"，常用来形容对长辈或上级的不当行为。