机密
jī mì · HSK 7
secret
🔒📄 机 crucial-mechanism + 密 dense/secret = a tightly-sealed secret matter, classified
机 here means a critical pivot or mechanism; 密 means dense, tight, hence secret. Together: information sealed too tight to leak. 密 mì also opens 秘密 (mìmì, secret) and 密码 (password), so 密 = the secrecy half.
密 — 宀 roof + 必 (phonetic) + 山 mountain
Originally referring to a hidden, sheltered place in the mountains, 密 combines 宀 (roof/cover) over 必 (phonetic) with 山 (mountain) at the base. The meaning extended from 'secluded mountain hideaway' to 'dense, close, secret'. Today it commonly means dense, intimate, or secret.