This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
膖 combines the flesh radical ⺼ with 逢, which likely contributes the sound. The exact historical development is uncertain.
The flesh radical ⺼ suggests a body part or condition, but the exact meaning is unclear. Think of it as a swollen or inflamed body part, based on the reading は.れる (to swell).
For ホウ, imagine a swollen body part making a hollow sound when tapped: hollow -> ホウ, and the flesh radical ⺼ reminds you it's a body condition.