襦 combines the clothing radical 衤 with 需, which likely contributes the sound and may relate to the idea of something needed or soft, but the exact historical development is uncertain.
Meaning mnemonic
The clothing radical 衤 beside 需 suggests a garment that is needed close to the body: underwear.
Reading mnemonic
For ジュ, imagine a soft undershirt made of a material that sounds like 'ju' when you rub it: ju -> ジュ, and it's your needed underwear.
