This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
塉 combines 土 (earth) with 脊, but the historical development and original meaning are uncertain.
The left side is earth 土, and the right side 脊 suggests a spine or ridge. Picture a barren ridge of earth where nothing grows.
For セキ, imagine a barren ridge of earth that is a sacred (セキ) place: sacred -> セキ, and the ridge is too poor to farm.