1. She had done her duty by him
  2. She did well by her children

The use of preposition 'by' in the above two sentences sounds to me like it means 'because of'. I coudn't find any meaning of preposition 'by' matching with what I feel. I'm not certain that 'by' in these two sentences means 'because of' or has a close meaning or something. So...

...What is the meaning of 'by' in this situation? Please give me some more examples so that I could get a bigger picture about this case!

  • I searched for the exact phrase you asked and found the answer online: forum.wordreference.com/threads/… – Bernhard Döbler Oct 28 '19 at 13:06
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    It's much the same as That's fine by me - where the preposition has the sense of about, concerning, with respect to, in regard to, as concerns (often, effectively interchangeable with for). – FumbleFingers Reinstate Monica Oct 28 '19 at 13:55
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    It can be used in that sense: 'By Pythogoras, we know that the hypotenuse of a right-angled triangle, with sides of length 3 and 4, is 5.' This says that because of Pythogoras' theorem we can calculate this. It's understandable, I think, but not that common. – simon at rcl Oct 28 '19 at 15:17
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    @simonatrcl: I'd say your Pythagoras example is just the very common by = via / through / using sense - it's just that at the semantic level it could also make sense with by = because (except in reality that's not implicit in the preposition - it's just in the context itself). – FumbleFingers Reinstate Monica Oct 28 '19 at 15:51
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    I would say that on behalf of is more equivalent than because of. – Jason Bassford Oct 29 '19 at 2:47

This is a specific, idiomatic use of "by".

She did right by him.

means that she did right /for/ or /to/ him, not that she did right "because of" him. She took care of him, fulfilled her obligations to him, was kind toward him, etc.

It has the sense of "If you asked him, he would say that she did right with respect to him."

This is also different than "on behalf of", which means something like "he asked her to do right (in some other context not involving him), and she therefore did so."

(In American English this use of "by" is less common on the coasts or in cities. I'm not certain about British English, but I've never heard a British speaker use it.)

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