Ain't - (pronoun) A slang compound word that usually is replaced by the compound word "isn't" meaning "is not". Wether is a world is debateable, but if were to be a word, this would be the most correct defination.
He isn't going to the store right now.
(this would me grammatically correct to say this)
He ain't going to the store right now.
(this isn't a correct word because usually a compound word takes out one letter on the word, ain't, if you tried to add a letter to ain't you more than likely wouldn't get anything. But ain't just seems to sound better in syntax.)