This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 侻 is uncertain. It combines 亻 (person) with 兑, but the original semantic and phonetic roles are unclear.
Since the meaning is unclear, focus on the shape: a person 亻 beside 兑, which looks like a container with legs. Imagine a person carrying a lightweight container easily.
For タツ, imagine a person (亻) easily lifting a container (兑) with a sharp 'tats!' sound: tats -> タツ.