[ahrnt, ahr-uh nt]
contraction of are not.
contraction of am not (used interrogatively).
Origin of aren't
as contraction of am not, a doublet of ain't (without raising of the vowel), spelling aren't by r-less speakers; ar was later substituted for the long a by speakers who regularly pronounce pre-consonantal r
The social unacceptability of ain't, the historical contraction of am not, has created a gap in the pattern of verbal contractions. I'm not, the alternative to I ain't, has no corresponding interrogative form except ain't I. In questions, ain't I is often avoided by the use of aren't I: I'm right, aren't I? Aren't I on the list? This aren't is simply a different outcome of the same historical development that yielded ain't, but the fact that it is spelled and pronounced like the contraction of are not (as in You are staying, aren't you? ) apparently gives it, for some, an acceptability that ain't lacks. The use of aren't I is objected to by others because a declarative counterpart, I aren't, does not exist. Many speakers, however, prefer aren't I to the uncontracted, rather formal am I not. See also ain't, contraction.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
informal, mainly British (used in interrogative sentences) am not
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
1794, contraction of are not, originally written are'n't and generally so into early 19c.
If "ain't I?" is objected to, surely "aren't I?" is very much worse. [Lady Grove, "The Social Fetich," 1907]
Online Etymology Dictionary, © 2010 Douglas Harper