You shouldn't talk to elders like that, it's disrespectful to them.

disrespectful 是由前缀 dis-（表示"否定、相反"）和 respectful（"尊敬的"）组成的形容词。respectful 来自动词 respect（"尊重"）加上形容词后缀 -ful（"充满...的"）。disrespectful 整体意思是"不尊重的、无礼的"。