日薄西山 rìbóxīshān
[declining rapidly as the sun is setting beyond the western hills;nearing one's end] 薄:逼近。太阳快要落山。比喻事物接近衰亡或人到老年,接近死亡
臣已日薄西山,余光无几,酬恩报国,正在斯时。——《宋史·赵普传》