This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
訞 combines 言 (speech) with 夭, which may contribute sound and possibly a sense of calamity or strangeness, but the exact historical development is uncertain.
The left side 言 suggests words, and the right side 夭 resembles a person with bent legs, hinting at twisted or strange speech. Picture words that are warped and ominous.
For ヨウ, imagine a yōkai (妖怪) whispering strange words: yō -> ヨウ, and the twisted speech feels eerie.