Informal or dialectal word meaning 'am not,' 'is not,' or 'are not,' common in many regional and working-class English varieties.
Possibly from Scottish and Northern English dialects, blending forms of 'am,' 'is,' and 'are' with negation; gained wider use through African American Vernacular English and continues to spread.
Linguists now recognize 'ain't' as a legitimate part of many English dialects, and it's been in use since at least the 1700s—what started as 'proper English' dismissing it shows how language prejudice ignores real grammar systems people use.
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