秘

Grain radical on the left - pictograph of a ripe rice stalk bending from the weight of its head. 秘 originally referred to a sacred grain offered in temple ritual, where secrecy and reverence overlapped. From 'sacred grain' came the modern 'secret, mysterious'. Same radical groups 秋 autumn, 种 seed, 稳 stable.

Right side 必 supplies the sound: bì → mì with regular b/m alternation in older Chinese. 必 also appears as phonetic in 泌 (secrete), 蜜 (honey), 谧 (quiet) - a tight phonetic family. Adds faint semantic weight too: 必 means 'must, certain', and a 秘密 (secret) is something that absolutely must not leak.