This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
馽 combines 馬 (horse) with 中 (middle), but the exact historical development is uncertain. It may have originally related to tying or hobbling a horse, but this is speculative.
The kanji shows a horse 馬 with something 中 in the middle, perhaps a rope or hobble. Without a clear meaning, picture a horse tied in the middle of a field.
For チュウ, imagine the sound of a horse chewing on a rope tied around its middle: chew -> チュウ, and the horse is restrained.