After his death, she became slightly unbalanced.

unbalanced 是由前缀 un-（表示"不、非"）和形容词 balanced（平衡的）组成的复合词。balanced 来自动词 balance（保持平衡），加上 -ed 变成形容词。 unbalanced 字面意思是"不平衡的"，在这里引申为"精神失常的"，指心理或精神状态失去平衡。