His poor health made him unfit for military service.

unfit 是由前缀 un-（表示"不、非"）和形容词 fit（"合适的、健康的"）组成的形容词，意思是"不适合的、不合格的"。 unfit for military service 表示"不适合服兵役"，指因身体条件不达标而无法参军。