autumn
龝 centers on the season of autumn, the time of harvest and falling leaves.
龝 combines 禾 (grain) with 龜 (turtle). The grain component points to harvest, while the turtle may have contributed sound or a seasonal association, but the exact historical development is uncertain.
The left side 禾 shows grain ready for harvest, and the right side 龜 looks like a turtle slowly moving through fallen leaves. Together they paint a picture of autumn.
For シュウ, imagine a shoe stepping on crisp autumn leaves: shoe -> シュウ, and the grain is being harvested.