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What's the meaning of the idiom "Do someone". I heard it in when two people were talking about love and one of 'em said:
"Yeah. A lot of good it's done us."
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This sentence actually means "It has not done a great deal for us"
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Let's back up:

It will do us good - it will be beneficial for us.
It's done us some good - it was beneficial for us
It's not done us any good - it was not beneficial for us

Sarcastic tone: A lot of good it's done us = It's not done us any good at all = it was not beneficial in any way