This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
秞 combines 禾 (grain) with 由, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side 禾 suggests grain or a plant, and the right side 由 looks like a sprout emerging. Imagine a grain plant sprouting vigorously, hinting at the kun-reading しげ.る (to grow thickly).
For ユウ, picture a young grain sprout growing thickly: young -> ユウ, and the grain plant 禾 with the sprout 由 becomes lush.