Like most vertebrates, human beings are more or less bilaterally symmetrical. Sure, you've got a mole on one shoulder and your liver is very much one-sided. But still, it's not as though we all had a lone tentacle growing out of a single hip. So how come left and right?
It's a fairly peculiar thing, when you think about it. Up and down are easy: gravity does the work, and besides the sky is blue. Forward and backward are likewise a piece of cake: we're iced on one side and not on the other. (We're bilaterally symmetrical in only one plane.) But right and left?
Until very recently we thought that left and right was a human category; that is, that there was no left and right in nature. However, in 1957 atomic physicists Wu, Hayward, Hoppes, and Hudson proved experimentally that if magnetized cobalt-60 atoms were spun counterclockwise (a "left, right" thing, note) they emitted radiation that preferred to go the other way; and, significantly, if the direction of spin was changed, the radiation stubbornly stuck with its preferred direction.
But most of us can't find our supply of cobalt-60, let alone get it to spin. So that deep natural truth can't be the origin of our concept.
We're told that our left brain hemisphere works differently from our right brain hemisphere (though what we read on left brain / right brain differences is mostly bunk); but just try sensing which hemisphere is doing what without a truckload of scanners. If you poke about or booze to excess you can, with some difficulty, find your liver there on your right side. (Ma foi!) So perhaps it's the heart, which is tucked just to the left of the midline — your left (which comes perilously close to introducing mirrors into this fillip) — that is the telltale, the significant deviant that allowed our species to develop the concept of right and left. Or maybe it was the duller fact that something more than 90% of us favour the right hand when it comes to throwing stones.
Whatever the origin, it's become second nature to us. Well, most of us. Because some folks find it devilishly hard to learn which way to turn, as in "No, your other left." If they use a Roman alphabet, the easiest mnemonic, I'd guess, would be the L that your left hand makes when held out in front of you ("No, your other front.") with the palm facing out and the thumb extended. If you're familiar with where you are and the landmarks, you might be able to get along with the compass points most of the time, turning "North" or "West" as the occasion demanded, rather than going three blocks before turning left. But try and find true north when you're buried in the bowels of a building.
Some people can, though. Lera Boroditsky, a Stanford psychology prof, writes that in some cultures everyone knows at all times which way is which; they actually say the equivalent of "Oh, I must have left my glasses to the southwest of the telephone." (By the way, her whole Edge talk on the Whorfian hypothesis is fascinating.) Incidentally, and speaking of cardinal points, if the L = left trick doesn't work for you, you can always find left and right with a compass provided you're not too close to the pole: according to Wikipedia, "Facing the sun, before noon, the north pointer of the compass points to the left hand. After noon, it points to the right."
All of this is just to get the concept functioning. Then the real fuss starts, which could be the subject of a good many future fillips. Having labelled the stone-throwing hand as right, every culture (seemingly) has privileged it. We don't need to go into the linguistic evidences, except perhaps to point out that we in law are big users of the distinction; that is, having made might right, we've abstracted it into, well, right. In some other cultures the matter is less abstract, because the left hand is relegated to latrine duty and, so, using it for much else would be beyond gauche. Poor left. But perhaps Captain Canuck can come to the rescue — or bring some solace, at least. Seems that Canadians use left-handed hockey sticks, by contrast with our righty American cousins. (Now that blades curve, like magnetized cobalt-60, sticks come marked with L and R.) No one knows why we do it. It's been called "a cultural quirk," but it might be something bigger, it might be the rise of the left.
No, the other left.