This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
誯 combines 言 (speech) with 昌, which likely contributes the sound. The exact historical development is uncertain.
The left side is 言 (speech) and the right side is 昌 (prosperous). Without a clear meaning, picture prosperous speech that is hard to pin down.
For ショウ, imagine a show of prosperous speech: show -> ショウ, even though the meaning is uncertain.