This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
跊 combines 足 (foot) with 未, but the exact historical development and original meaning are uncertain.
The left side 足 suggests a foot, and the right side 未 looks like a tree with extra branches. Imagine stepping on a fallen branch, but the meaning remains unclear.
For バイ, picture stepping on a branch and saying 'bye' to your balance: bye -> バイ.