tuck into one's obi
襭 centers on the action of tucking something, especially fabric or an object, into the fold of an obi (sash) to secure it.
襭 combines the clothing radical 衤 with 頡, which likely contributes the sound. The character is associated with the action of tucking into clothing, but the detailed historical development of the right-side component is uncertain.
The left side 衤 (clothing) and the right side 頡 (which looks like a head with a page-like element) can be imagined as tucking a written page into the fold of a robe: 襭 means to tuck into one's obi.
For ケツ, imagine tucking a ketchup packet into your obi: ketchup -> ケツ, and the packet is secured in the sash.