go downhill; be on the decline; 走下坡路
At that time our relations were going upwards, but since then they have been going downwards [downhill]. 那时我们的关系是走上坡路, 此后就开始走下坡路了。
牛津高阶第八版
美国韦氏词典
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维基百科 (自由的百科全书)