quiet, calm, tranquil, peaceful
忪 centers on a state of inner stillness and peace: quiet, calm, and tranquil. The given meanings all describe a serene, undisturbed condition.
忪 combines the heart radical 忄 with 公, which likely provides the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound where the heart relates to the calm, quiet meaning.
The heart radical 忄 beside 公 (public) suggests a heart that is open and at ease in public, perfectly calm and quiet.
For ショウ, imagine a quiet, peaceful show where the audience sits in calm silence: show -> ショウ, and the heart is tranquil.