Is this phrase grammatically correct?
Is you is or is you ain't my baby?
It's from a Tom and Jerry cartoon: http://vimeo.com/40283242 (at 1:30, 2:00 and 3:00).
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The Wikipedia article says "The phrase 'Is you is or is you ain't' is dialect, apparently first recorded in a 1921 story by Octavus Roy Cohen, a Jewish writer from South Carolina who wrote humorous black dialect fiction."
It's not grammatical standard English, but it's not intended to be. It's a song lyric.
The link to the blog post about grammaticality in Stan's comment is a really good one, so I'm copying it for this answer.