This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
脺 combines the flesh component ⺼ with 卒, which likely contributes the sound. The exact historical development is uncertain.
The left side ⺼ suggests flesh or body, and the right side 卒 looks like a soldier. Picture a soldier's body, but the meaning remains unclear.
For セイ, imagine a soldier saying 'say' as he points to his flesh: say -> セイ, though the kanji's meaning is uncertain.