This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 兊 is uncertain. It appears to be a variant or related form of 兌, but its origin is not well documented.
Since the meaning is unclear, focus on the shape: 八 (eight) over 允 (a person with a bent leg). Picture a person with eight legs, a strange and memorable image.
For ダ, imagine a person with eight legs trying to dance: the first sound of 'dance' is da, cueing ダ.