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1. Contraction of am not.
2. Used also as a contraction for are not, is not, has not, and have not.
Usage Note: Ain't has a long history of controversy. It first appeared in 1778, evolving from an earlier an't, which arose almost a century earlier as a contraction of are not and am not. In fact, ain't arose at the tail end of an era that saw the introduction of a number of our most common contractions, including don't and won't. But while don't and won't eventually became accepted at all levels of speech and writing, ain't was to receive a barrage of criticism in the 19th century for having no set sequence of words from which it can be contracted and for being a "vulgarism," that is, a term used by the lower classes, although an't had been originally used by the upper classes as well. At the same time ain't's uses were multiplying to include has not, have not, and is not, by influence of forms like ha'n't and i'n't. It may be that these extended uses helped fuel the negative reaction. Whatever the case, criticism of ain't by usage commentators and teachers has not subsided, and the use of ain't is often regarded as a sign of ignorance. · But despite all the attempts to ban it, ain't continues to enjoy extensive use in speech. Even educated and upper-class speakers see no substitute in folksy expressions such as Say it ain't so and You ain't seen nothin' yet. · The stigmatization of ain't leaves us with no happy alternative for use in first-person questions. The widely used aren't I?, though irregular, was found acceptable for use in speech by a majority of the Usage Panel as long ago as 1964, but in writing there is no acceptable substitute for the stilted am I not?
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
am not, is not, are not, have not, or has not: I ain't seen it.
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
1. Nonstandard except in some dialects. am not; are not; is not.
2. Nonstandard. have not; has not; do not; does not; did not.
[1770–80; variant of amn't (contraction of am not) by loss of m and raising with compensatory lengthening of a; compare aren ' t]
usage: As a substitute for am not, is not, and are not in declarative sentences, ain ' t is more common in uneducated speech than in educated, but it occurs with some frequency in the informal speech of the educated, esp. in the southern and south-central states. This is especially true of the interrogative ain't I? used as a substitute for the formal am I not? or for aren't I? (considered by some to be ungrammatical) or for the awkward amn't I? (which is rare in American speech). Some speakers avoid all of the preceding forms by substituting Isn't that so (true, the case)? ain ' t occurs in set phrases: Ain't it the truth! The word is also used for emphasis: That just ain't so! It does not appear in formal writing except for deliberate (often humorous) effect or to represent speech. As a substitute for have not or has not and - occasionally in Southern speech - do not, does not, and did not, it is nonstandard except in similar humorous uses: You ain't seen nothin' yet! See also aren't.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.