This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 徰 is uncertain. It visually combines 彳 (movement) with two stacked 正 (correct), but this may be a variant or non-standard form.
With no clear meaning, picture a person 彳 walking while carrying two stacked 正 marks, like a mysterious double-correct sign on a journey.
For セイ, imagine saying 'say' to the double-correct traveler: say -> セイ, as you call out to the walking figure.
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