A cubit is an old unit of length from a time before metres. People used the body: the forearm. In the Bible and in ancient building, the cubit is usually from the elbow to the tip of the middle finger, often about 45 to 52 centimetres, though it was never one exact world-wide size because arms are not all the same. Learner dictionaries sometimes say “wrist to elbow,” which is the same general idea — the lower arm — but a little short of the old cubit, which ran on to the fingertips. You almost never hear the word in ordinary talk now, except in history, the Bible, or when someone is being playful about old measures.