This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
滭 combines 氵 (water) with 畢, which likely contributes the sound. The exact historical development is uncertain.
The water radical 氵 beside 畢 suggests something related to water, but the specific meaning is unclear. Imagine a rare, bubbling spring with an unpronounceable name.
For ヒツ, picture a hidden hot spring (ヒツ) where water bubbles up: the invented name 'Hitsu' cues ヒツ, and the water radical 氵 ties it to the spring.