This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
緆 combines 糸 (thread) with 易, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side 糸 suggests thread or cloth. The right side 易 looks like a simplified container. Picture a fine thread being pulled from a container, but remember the meaning is uncertain.
For セキ, imagine a thread so fine it is almost invisible, like a single seki (セキ) stone in a vast plain. The thread is as small as a pebble.