everyday clothing, underwear
衵 centers on clothing worn close to the body in daily life: everyday garments and underwear.
衵 combines the clothing radical 衤 with 日, which likely acts as a phonetic component. The exact historical development is uncertain.
The clothing radical 衤 beside 日 (sun/day) suggests clothes you wear every day, like underwear that sees the light of day.
For ジツ, imagine putting on your everyday clothes in a jiffy: jits -> ジツ, and you're ready for the day.