pleat, fold, tuck, crease
襞 centers on a deliberate fold or crease in fabric, such as a pleat, tuck, or gathered wrinkle. All given meanings describe the same physical action of doubling material over itself.
襞 combines 辟 (which likely contributes the sound) with 衣 (clothing), suggesting a fold or pleat in fabric. The exact historical development of 辟 in this context is uncertain.
The top 辟 looks like a complex folded shape, and the bottom 衣 is clothing. Imagine folding a piece of clothing into neat pleats: 襞 is that fold or crease.
For ヘキ, picture a tailor making a sharp pleat and saying 'heck!' as they press it down: heck -> ヘキ, and the fold stays crisp.