His immature behavior during the meeting was unprofessional and annoyed everyone.

immature 是由前缀 im-（表示"否定"）和 mature（成熟的）组成的形容词。mature 来自拉丁语 maturus，意思是"成熟的、发育完全的"。immature 字面意思是"不成熟的"，引申为"幼稚的、不成熟的"。