real, genuine, actual
寔 centers on the idea of something being real, genuine, or actual, as opposed to false or merely apparent.
寔 combines 宀 (roof) with 是, which likely contributes both sound and the idea of correctness or reality. The exact historical development is uncertain.
Under a roof 宀, what is 是 (correct, right) is what is real and genuine. Picture a house where only true things are kept.
For ショク, imagine a real, genuine shock when you discover the truth under the roof: shock -> ショク.