The shape that pops up on both sides of 秘密 (himitsu: secret, to keep secret + secret) is 必 (kanara(zu): necessary). Henshall says 必 means “necessarily” in both characters and acts phonetically to express a meaning. But … that phonetically expressed meaning differs in each word!
Literally, he says, 必 represents an “encased halberd,” which is to say (I think) a sword in a sheath (though it looks more like a sword plunging through someone’s heart!).
In 密, the 必 acts phonetically to express “comb,” also lending an idea of being tightly packed.
And in 秘, the 必 phonetically expresses “to hide,” while the literal meaning probably lends the connotation of “concealment.” Meanwhile, the rice plant (禾) on the left was originally 示, an altar, and most likely turned into rice through miscopying. So the whole character meant “hidden things of the gods,” which is to say “mystical secrets,” now construed more broadly as “secret.”
