This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
坧 combines 土 (earth) and 石 (stone), but its exact historical development and original meaning are uncertain.
The left side is earth 土 and the right side is stone 石. Picture a foundation stone set into the earth, though the kanji's actual meaning is unclear.
For シャク, imagine a shaky stone foundation: shaky -> シャク, and the earth and stone shift beneath it.