Let's start 2 or 3 Letter Tuesday at the very first word in all Scrabble lists: AA. Most Scrabble players know it because they've played it or seen it played. But what is it?
Let's start with pronunciation. It's something like AAH that stops short rather than trailing off. It's not ay-ay or ah-ah (which is what I thought all this time til I hit the pronunciation button on the Collins' entry just now.
As for what it is, it comes from the Hawaiian language and describes a kind of language (kind of like people of cold climates having words for types of snow, Hawaiians have words for different types of lava).
Aa is a slow-moving lava, and the surface of it therefore keeps cooling and then breaking up again as the lava keeps moving. It moves as a noisy tumble of hot rocks rather than a smooth flow. When it cools entirely, it forms very jagged, sharp rocky surface. You don't want to walk across a field of aa lava, even long after it is cooled, because the rocks are often loose and it's all too easy to take a tumble -- and it that happens, you will get cut up because the rocks are so sharp. It's no fun.
Here's a video showing flowing aa:
Since there isn't much more to say about aa, I'll just quickly mention another good Hawaiian word to have in your vocabulary: pahoehoe. This is the other kind of lava most often seen in Hawaiian volcano eruptions, and is a smooth, faster flowing lava. In fact, when the camera in the above video pans to the left, you can see that the current aa flow is covering an old hardened flow of pahoehoe.
Here's what active pahoehoe looks like:
Pahoehoe is also a legal Scrabble word, by the way!