water

he treated it as water off a duck's back  

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of no importance  

 He was told that the new plan he'd developed just wouldn't hold water

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wouldn't work 

Every time we came up with a new proposal, he would throw cold water on it    

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discourage it 

After all the money that she's borrowed and also spent, she now finds herself in deep water   
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in defeat

  He's decided to make a fresh start and regards all his old problems as water under the bridge  

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not worth worrying about 

  Once he's earned a bit of money he immediately spends it like water   

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spends it freely 

she always feels like a fish out of water 

====================== 

uncomfortable
 

It's many years since we last met and a lot of water has passed under the bridge since then  

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a lot has happened
 

 The jewels in that showcase are clearly of the first water

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of high quality 

Their job as advisers was to pour oil on troubled waters 

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calm things down