warning, counsel, presept, needle
箴 centers on a pointed corrective message: a warning, counsel, or precept that pierces like a needle to guide behavior.
箴 combines 竹 (bamboo) and 咸, which likely contributes the sound and may relate to a piercing or admonishing sense. Historically, bamboo was used for needles or writing slips, fitting the needle and warning meanings.
Bamboo (竹) can be sharpened into a needle, and the bottom part (咸) looks like a mouth with a weapon, as if a sharp verbal warning is being given.
For シン, imagine a thin bamboo needle giving a sharp warning: thin -> シン, and the needle pierces the conscience.