”I don't know nothing”? ”It ain't no lie”?

strictly speaking, both are these sentences are grammatically wrong, they are more slang than proper English.

this kind of street talk usually replaces "any" with "no",
e.g.
I don't know anything => I don't know nothing
It isn't any lie => It ain't no lie

this, however, does not change the meaning, so it's still negative.

this source of this pattern may be from immigrants whose original languages do contain such grammar, e.g. Italians.

In English, there is no double negative. Double negative does not mean positive.

It only serves to stress the meaning.

2008-07-12 17:13:31 補充：
只是加重語氣，不是負負得正。