This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 軃 is uncertain. It combines 身 (body) with 單, but the original meaning and component function are unclear.
The kanji combines 身 (body) and 單 (simple/single), but without a clear meaning, it is hard to form a reliable mnemonic. You might imagine a single body, but this is speculative.
For タ, picture a single body (身) standing alone (單) and saying 'ta!' as a short, simple sound: ta -> タ.
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