秘密
mìmì · HSK 4
secret
🤫 秘密 = 秘 (secret) + 密 (close/dense) — doubled for emphasis
Both characters mean secret/hidden on their own; stacking them intensifies the meaning into a proper kept secret. 密 also shows up in 密码 (password) and 紧密 (tight-knit), always carrying that sense of closed-off density.
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.