Ain't ain't a word, and you ain't gonna use it.
This is a line I remember from my childhood, a line that embodies the odd feelings we English-speakers have towards this little word. This sentence is itself amusing, as it quite plainly contradicts itself, bringing to mind the self-referential sentences like This sentence is false explored by, among many others, Douglas Hofstadter (the reader is referred to the book Gödel, Escher, Bach: An Eternal Golden Braid.) But I digress. This usage commandment is quite a witty little line, as the contradiction therein illustrates so well people's feelings about the word: it is a word, as is evidenced quite plainly by its use in the sentence — and yet it ain't, and it certainly ain't appropriate for anyone with any modicum of intelligence or education. You are commanded not to use the word ain't — and yet it's common in many English dialects and most people use it some of the time.
As a matter of fact, ain't originally played the role of a contraction for am not; nothing has yet arisen to replace it in standard English. In a statement, the speaker still has a useful alternative: instead of using this lexical outlaw, contract I am not to I'm not. But in a question, the options ain't as pretty: either refrain from using contractions entirely, and thus use the stilted-sounding full version Am I not?, or else the rather illogical Aren't I?, which suggests the plainly ungrammatical *Are I not? as a full form. Sojourner Truth made a famous speech in which she asked, "Ain't I a woman?" The reader who doubts my analysis may confirm it by attempting to find a suitable rephrasing for her question that fits the rules of Standard English.
At this point, my more astute readers are wondering why the English teachers of the world have decided to condemn this useful little word. After all, it has perfectly acceptable companions for the other persons, aren't and isn't. (My less astute readers are advised that extended warranties on consumer goods are rarely a good investment.) Ain't doesn't seem, on the surface, any less decent than its colleagues; further, it obviates the need for such barbarisms as "Aren't I a woman?" So let's see what can be gleaned from its history.
The troublesome part is that the word's origins are not known with any certainty. The oldest known usages of contractions involving n't date back to the 1660s, where they appeared in comedies by Dryden and others. Words like isn't and doesn't leave no doubt as to their origins. Don't and won't don't correspond as directly to their corresponding full forms, do not and will not, but they have been unambiguously traced to particular English dialects. Ain't, however, ain't as transparent: ai is obviously not an English verb, and it ain't similar enough to any English verb to give us much guidance.
It has been suggested that ain't developed from dialectal forms of either am not or are not. The former suggestion relies upon the intermediate form amn't (rare, but still found in some English dialects); if the /m/ disappeared because it was followed by another nasal consonant, this would produce an't (also uncommon today, but attested in literature around the same time as ain't) which might conceivably have turned into ain't. The other proposed origin supposes that are not was first shortened to aren't; the /r/ sound might then have been lost, as they regularly are at the ends of syllables in modern British English. That would have left a form that I'll spell aan't, sounding like the word aunt (for those who don't pronounce it like the insect.) Again, then, we have to guess at some change in the vowel to the one present in its modern pronunciation.
It has also been suggested that it originated from have not. The loss of that /v/ ain't a surprise (compare the once common o'er and e'er for similar examples); have was once pronounced as in behave, making the vowel no particular problem in this scenario, and the various disappearance and reappearance of /h/ in some varieties of English is well known. Think of Audrey Hepburn in My Fair Lady, vexing Rex Harrison with "In 'ertford, 'ereford and 'ampshire 'urricanes 'ardly hever 'appen". But all of these scenarios are just guesses.
Compounding the difficulty in the analysis is that ain't is used in place of several long forms — are not, is not, am not, have not, and has not (as in He ain't eaten dinner yet). Ain't was used for are not, is not, and am not from its earliest days; there's no way to guess at one origin or another based upon its original usage. Thus the precise origin of the word ain't certain; we can only propose possibilities.
Any fool can make a rule and any fool will mind it
Henry David Thoreau
So why does ain't piss people off, anyway?
At this point, my readers (both the astute and the less so) are waiting for me to get to get to some sort of goddamn point already. The question on all of our minds, then, is, "Why the fuck does every English teacher in the world have such a bug up their ass about ain't, anyway?" (The follow-up question, "Wait, should I use their as a gender-neutral possessive pronoun?", will have to wait for another day.)
The funny part is no one's really certain about that, either. It seems that people started objecting in the nineteenth century, which is a time when many prescriptive grammar rules — that is, those that insist upon certain usages rather than simply describing the usages that exist — first rose to prominence. It's well-known that, at least until the twentieth century, ain't wasn't particularly a marker of low-class or non-standard speech; nowadays, it's safe to say that the word ain't part of either the American or British standard dialects, though of course many intelligent, well-educated speakers of non-standard dialects use it in certain circumstances. It seems that it was quite a normal word for most people to use until wickedly misinformed grammar teachers started inveighing against its use. And once the rule was invented, as with many such rules, it developed a life of its own. Once the avoidance of ain't became a marker of "proper" speech, naturally no one who wished to speak properly would use it — regardless of how silly the rule was in the first place. (A similar thing happened with the grossly-misnamed phenomenon of the "split infinitive", which you still don't see much of in formal writing.)
The only theory I've run into explaining this prohibition is that ain't was noted for being illogical — that is, since it could replace so many different words in English (is not, are not, have not, and so forth), it was decided that the word was inherently lazy and a sign of degraded English. (Never mind the fact that the word didn't seem to cause any communication problems for those who used it.) Rather than taking the much more logical step of teaching that it should only be used to mean am not (still rather silly, in my book, but a tad more logically defensible) it was decided that the public couldn't be trusted with the word at all. Mismatched verbs are dangerous to society, you know. (Readers are no doubt aware of the well-established connections between the word ain't and drinking, drug use, and buggery.)
It ain't over till it's over.
So there ain't nothing wrong with ain't?
Of course not. Certainly it ain't part of the English standard anymore — but then, most people don't speak Standard English all the time. You don't have to listen too hard to find well-educated speakers using ain't in casual environments. Naturally, linguists have examined the use of contractions of verbs plus n't; they're the unmarked forms, which means they're the most common and ordinary way to express their particular concepts (whereas the full forms like am not are mostly reserved for extremely formal speech, and therefore highly marked). And while not takes stress in the full forms, the contractions don't require it, which makes them a bit more rhythmically natural in many cases — hence the odd quality of speech that lacks contractions.
Ain't has acquired some additional color, probably as a result of its condemnation. It is, of course, frequently used in dialogue in books and plays as a quick signal of someone's social origins. And even people who don't use the word normally sometimes use it for a folksy sound, or else because it seems more emphatic, probably from the notion that folksy means earthy and, as the kiddies say, "real". (Interestingly, while I can't seem to find any quotes in which our affectedly folksy president uses ain't, a Google search finds numerous examples of people using it in parodies of him.) A little bit of a violation of the grammar rules we were taught can lend a certain intensity to what a person says. No doubt this is a giddy little thrill for those poor souls suffering on without enough drinking, drug use, or buggery to entertain them. While the lack of a word like ain't in Standard English is sometimes a difficulty, I suppose I would hate to deny the joy that making and then violating arbitrary rules brings to those people's lives.
Stevens, Martin, 1954. "The Derivation of 'Ain't'". American Speech, 29.
Liberman, Anatoly, 2006. "The Much Villified Ain't'". From his column, The Oxford Etymologist, which appears in the Oxford University Press blog. (http://blog.oup.com/oupblog/2006/08/the_much_vilifi.html)