Why people say "I didn't do nothing"?
I think the meaning he wants to express is "I didn't do anything" or "I do nothing". Why he uses "not do nothing"?
Any comment is appreciated.

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Answer

Very common in England as a way of emphasising it wasn't your fault " I din do nuffin!"

Answer

It should really be "I didn't do anything" or "I DID nothing".

Unless the speaker intended to use a double negative (ie. "He did something"), then "He didn't do nothing" is incorrect and and is considered substandard English by native speakers. It's only used in some parts of the US in the same places that also use "ain't".

Answer

because they were using Ebonics English