This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
泬 combines 氵 (water) with 穴 (hole/cave). The exact historical development is uncertain, but it likely relates to water emerging from a hole or spring.
Water 氵 beside a hole 穴 suggests water gushing from an opening, but the actual meaning is unclear.
For ケツ, imagine water gushing from a hole with a sharp 'kets' sound: kets -> ケツ.