This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 斊 is uncertain. It visually combines 文 and 耳, but the original component relationship is unclear.
With 文 (writing, pattern) over 耳 (ear), you might imagine a pattern heard or a written ear, but the true meaning is unknown.
For セイ, imagine a sage (セイ) trying to decipher the mysterious pattern 文 with their ear 耳, but even the sage is unsure.