Ain't is a negative inflection for am, is, are, has, and have in informal English. In some dialects, it is also used for do, does, did, and will. The development of ain't for the various forms of be, have, will and do occurred independently, at different times. The use of ain't for the forms of be was established by the mid-18th century and for the forms of have by the early 19th century.

The use of ain't is a continuing subject of controversy in English. It is commonly spoken in informal settings, especially in certain regions and dialects. It is often highly stigmatized and is often understood as a marker of low socio-economic or regional status or education level. It is generally considered non-standard by dictionaries and style guides except when used for rhetorical effect.