A is for...

One of two indefinite articles in the English language, the other being an. According to the Online Etymology Dictionary, it used to be that an was the sole indefinite article, but folks had mostly switched to only using an in front of words beginning with vowel sounds by the 14th century. Sometimes you'll hear snooty intellectual types use an before words that begin with h, as in an historic event. This usage comes from the fact that proper folk used to drop the h sound at the beginning of some words (an 'istoric event). This rule, then, would only apply to Cockneys and such.
An, as an article, comes from the Old English word an (pronounced ane, I think), which means one. Old English, however, didn't use indefinite articles; they only used an to specify one. Why, do you suppose, we have indefinite articles at all? You don't need them, really. Is the sentence I want apple less clear than I want an apple? If you wanted more than one apple, you would say apples, and if you wanted a specific apple, you'd say the apple or that apple. According to Wikipedia, linguists believe that the Proto Indo-European language (great grandparent of languages as varied as English, Greek, and Sanskrit, Hindi, and many others) did NOT use articles - definite or otherwise, nor did many of its immediate descendants (Latin, Persian, and Sanskrit, for example). Yet this quirk of language isn't just one of the many inane qualities of the English language - many languages use articles, and many languages' indefinite articles descend from some word that once meant one.
I've talked about Indo-European before, because years and years after learning about it, it still fries my noodle. Finding out English and Hindi are related felt like me finding out that I was related to Samuel L. Jackson. 

...and a spigot. Because I think myself deep and artistic.

Know what else blows my mind? We don't have a single surviving example of the language known as Proto Indo-European (far as I know), we only know it existed because of its offspring, and we can guess what its vocabulary was like by looking at the similarities between its offspring. Doesn't that just make your head explode? Or is that just a word nerd thing?