This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 萯 is uncertain. It combines the plant radical 艹 with 負, but the original semantic and phonetic roles are unclear.
The top 艹 suggests a plant, and the bottom 負 can mean 'bear' or 'carry'. Imagine a plant that bears a heavy fruit, like the からすうり (karasuuri) gourd, but the exact meaning is uncertain.
For フウ, imagine a strong wind (風, ふう) blowing through the grass top 艹, carrying the plant's seeds: 風 -> フウ.