neck of a garment, gusset, gore
衽 centers on parts of a garment near the neck or chest: the neck opening, a gusset (a triangular insert for shaping), or a gore (a shaped panel). These are all structural fabric pieces that shape the upper part of clothing.
衽 combines the clothing radical 衤 with 壬, which likely contributes the sound. The character refers to garment parts, consistent with the clothing radical.
The clothing radical 衤 on the left shows this is about garments. The right side 壬 looks like a person wearing a robe with a shaped neckline. Picture a tailor adding a gusset or gore to the neck of a garment.
For ジン, imagine a tailor named Jin who specializes in sewing necklines and gussets: Jin -> ジン, and his needle shapes the garment.