This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 洯 is uncertain. It visually combines 㓞 (a component associated with cutting or carving) with 水 (water), but the original meaning and formation are unclear.
The top 㓞 suggests cutting or carving, and the bottom 水 is water. Imagine water carving a channel, but remember the meaning is uncertain.
For ケツ, imagine a kettle (ケツ) of water being carved from stone: kettle -> ケツ, and the water 水 flows out.