sag [sæɡ]

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vi.下垂，下跌，消沉；vt.使下垂；n.下垂，下跌，凹陷，[航海]随风漂流

to drop down to a lower level in the middle

The shelf was beginning to sag beneath the weight of the books upon it. 书架在书的重压下渐渐下弯。
We need to do something about the sag. 我们须把下沉的地方修整一下。