sleeve, ample flowing robes
褎 centers on the sleeve of a garment, and by extension, the flowing, ample robes that feature such sleeves.
褎 combines 衣 (clothing) with a component that likely contributed the sound and the idea of a sleeve or flowing garment. The exact historical development of the inner part is uncertain.
The outer 衣 wraps around the inner shape like a robe. Picture the inner part as a grain plant 禾 peeking out from a wide sleeve, suggesting the flowing cloth of a sleeve.
For シュウ, imagine a flowing sleeve catching the wind with a soft 'shoo' sound: shoo -> シュウ, as the ample robe billows.