Short for "am not" and for nothing else – not even "is not", for "it ain't" would just be "it am not". The only correct use is "I ain't" which is a contraction of "I am not". So, don't say e.g. "you ain't" which would only mean "you am not".
Clinique: "Let 'im go, he didn't do nothing!"
TyRone: "I ain't done nothing! I ain't done no nothing!"
LaKeisha: "You bastards, let us go! I ain't done none!"
Barackeisha: "You let 'em go!! They's ain't have done nothing! They ain't have done none!"
As you read, there is no record of two pretty best friends. One of is always butt ugly while the other is hot.
Nah, Valentina and Sophia can't both be pretty cause i ain't never seen two pretty best friends, it's always one of them gotta be ugly. As you can see, Sophia is sexier.
What the manager of an antiques-restoration shop specializing in post-Renaissance-era pieces tells a trainee while surveying a recently-delivered assortment of vintageart and furniture.
To ensure that we don't become overwhelmed with antiques that need TLC, I always tell my repairmen to just stick to pieces dating from the early 1600's through the mid-1700's --- "If it ain't Baroque, don't fix it, boys --- we'll send all the items from other periods elsewhere for refurbishing."