The word "秘密" is formed through a process of semantic compounding, where two characters with closely related meanings are combined to reinforce a single, more nuanced concept. The first character, "秘", carries connotations of something hidden, mysterious, or withheld. The second, "密", adds the dimensions of being tight, dense, intimate, or secluded. By joining them, the compound word "秘密" synthesizes these ideas to fully express the notion of a "secret", something deliberately kept concealed, private, and closely guarded.