decline, go down, sunset
昃 centers on the sun moving downward from its peak: declining, going down, or the time of sunset.
昃 combines 日 (sun) with 仄, which likely contributes both sound and the idea of slanting or declining. The character depicts the sun tilting westward.
The sun 日 sits above 仄, which looks like a person leaning or slanting. Picture the sun leaning over as it begins to set.
For ショク, imagine the sun declining after a heavy shock: shock -> ショク, and the sun goes down.