This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 跀 is uncertain. It visually contains 足 (foot) and 月 (moon/meat), but the original component relationships are unclear.
The left side suggests a foot (足), and the right side is 月 (moon or flesh). Without a clear meaning, picture a foot that is somehow misshapen or twisted, as hinted by the reading ゆが.む (to warp).
For ゲツ, imagine a foot getting twisted into a weird shape, making a 'getsu' sound as it warps: getsu -> ゲツ.