sleeve, ample flowing robes
褏 centers on the sleeve of a garment, especially the wide, flowing sleeve of traditional robes. The meaning extends naturally to the ample, flowing robes themselves.
褏 combines 衣 (clothing) with 由, which likely contributes a phonetic element. The character is associated with sleeves and flowing garments, but the exact historical development is uncertain.
The clothing enclosure 衣 wraps around 由, like a sleeve enveloping an arm. Picture a wide, flowing sleeve of a robe, with the inner 由 as the arm passing through.
For シュウ, imagine a flowing sleeve making a soft 'shoo' sound as it moves through the air: 'shoo' -> シュウ, and the sleeve billows gracefully.