秘
ヒ, ひ.める, ひそ.か, かく.す

secret, conceal

JLPT1 G6 S10 F862

祕 is as per 必# (split) + 示 altar/the supernatural → *mysterious*/*secret* ritual involving actual/figurative and permanent splitting. 秘, which substitutes 禾 grain/rice for 示, is a variant form that has supplanted 祕. The use of 禾 may owe to connections with harvest rituals.

[Full Kanji Etymology List]