He was completely unconcerned about the weather change and still went out in a T-shirt.

unconcerned 是由前缀 un-（表示"不、非"）和形容词 concerned（关心的、担忧的）组成的形容词，字面意思是"不关心的"。concerned 来自动词 concern（涉及、使关心），加上 -ed 变成形容词。所以 unconcerned 表示"不关心的、不在意的"。