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the sentence "I didn't do nothing" means
1. I did do it?
2. I did not do it?
which one is right?

and where does this usage come from?
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"I didn't do nothing" is a double negative, probably with American origins. The phrase involves a contradiction of the intended meaning, and should be worded so:

"I didn't do anything" or
"I did nothing"

So (2) is the correct answer.
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It means "I didn't do anything' or 'I didn't do it'. Although it's grammatically incorrect, a lot of Americans use it in spoken English.
It gets used in England as well, but is considered a bit 'common'.
But aren't there instances where "I didn't do nothing", at least in some form, is grammatically correct?

For example,

Person A: You did nothing all day!
Person B: I most certainly didn't do nothing all day!

In this case "I didn't do nothing" means "I did something".
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Yes, as with most things, context makes it clear what the speaker means.
matthewg"I didn't do nothing" is a double negative, probably with American origins. The phrase involves a contradiction of the intended meaning, and should be worded so:

"I didn't do anything" or
"I did nothing"

So (2) is the correct answer.
Gosh, do you think that Chaucer and Shakespeare were American, too? Emotion: wink