Another Used To Be Two Words: An Other
So, aside from a-whole-nother being an isolated case, how else do we know that another is getting rebracketed? Well, in a more general sense, although another appears to be a single word, it also looks suspiciously like a fused form that may at one point have been two words: an other. So, we hypothesize that, at some point in the past, writing conventions were to spell another as two words: an other. (Spelling has become pretty standardized now, which is a good thing, but it was not so long ago that spelling and punctuation were in flux, in the United States and elsewhere. Here is one example: As recently as 1930, in an original-print hardcover of a novel in the Nancy Drew series, tomorrow is spelled to-morrow! Remember when books even older than that used expressions like on the morrow?)
Over time, re-analysis of the word boundary, probably because speakers seldom put anything between an and other, led to spelling conventions changing. Intuitively, we all agree that an and other are both real words that we use elsewhere all the time. Also, another can only be used to introduce singular nouns (e.g., we can’t say another girls), which means that there is an “indefinite-article feel” to the an in another. Another way of saying this is that the an inside another still has the meaning and function of separate indefinite article an.
And, guess what? The Oxford English Dictionary confirms this suspicion that another was technically two words in the recent past, in its etymological listing for another. They report that, originally, the an was separate from the other, and at different times, it was found both with the a separate from the nother, and even (less often) as two words with no “n” at all, like a other. They also confirm that another can be summarized as meaning a second (one), a remaining (one), or a different (one).
[Added 6/20/2014: See line 15 in this old manuscript for an example of a nother. Hat tip to Matt Gordon.]
This means that although we spell another as a single determiner, the string another friend [determiner + noun] could theoretically and accurately be reanalyzed—in terms of its meaning—as the three-word sequence [determiner + adjective + noun], parallel to the three-word sequence an intellectual friend.
In Speech, Breaks Between Words Are an Illusion
How do these word-boundary changes happen? Well, word boundaries are fascinating, because acoustically, human speech is a continuous stream of sound, and the spaces that we “hear” between words are an illusion. For most of human language history, there were no writing systems, which partly explains why children learn to speak and understand language at a very early age, quickly and effortlessly, by mere exposure, but yet must laboriously learn to read and write in school over a period of many years. The point here is that word boundaries are not always as natural, obvious, or easy to define as you might imagine. For example, as we add new words to the lexicon, especially compound words like website, we often start out spelling them with a space (web site), and then over a short time we “reanalyze” them as a single, compound word with no space. But, underlyingly, is it one word or two? How do we know? Really, we don’t; we just go by the convention.
In fact, studies have shown that very young children, who can’t yet read or spell, often analyze word chunks like an apple or the monkey as one single word. Why? Well, primarily because articles like an and the are determiners, which are those “function” words, and they come before apple and monkey, which are those “content” words. If asked to provide a word, any word, you are not likely to think of saying a function word like “the.” So, these function words—like the and an—that do not provide true meaning, but do serve a grammatical function, and, usually appear right before content words, are likely to be involved in the shifting of word boundaries, and/or spelling conventions, over time.
Another fascinating aspect is the fact that nearly all cases of nother are spoken aloud, right? Can you imagine typing out nother, even in an informal e-mail? The spelling convention would probably prevent you. So, the fact that we almost exclusively hear this phenomenon makes a lot of sense, because that is how it has managed to override our spelling conventions, just like the way those young children who haven’t yet learned to read sometimes analyze behave as [verb + adjective], be and “-hayve”, instead of as just a [verb], and, like the way any adults who think it’s “pullet surprise” (and there are a few such adults) have certainly simply never seen the name of the award written out before.
(Well, mostly, except in cases of intentional mockery, like this old cigarette ad. Notice how they used an apostrophe to indicate the break in the word!)