After months of caring for the newborn, she looked completely worn-down.

worn-down 是由动词短语 wear down 的过去分词形式构成的形容词。wear 意思是"磨损、消耗"，down 表示"向下、减弱"的状态。worn-down 合起来形容人因长期劳累而"精疲力竭"的状态，就像物体被不断磨损一样。