secret, dense

密 is composed of 宓 (silent) and 山 (mountain). 宓 conveys the idea of an extremely enclosed place, and its original meaning is to hide or to be crowded. However, as 宓 developed other meanings, the character 密 was separately created to take its place. Mountains are far from human habitation, and layers of overlapping peaks resemble a dense gathering of many people, further reinforcing the meanings of concealment and crowding in 密.