This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
粞 combines 米 (rice) with 西, but the historical development and original meaning are uncertain.
The left side is rice 米 and the right side is west 西. Picture a rare, forgotten grain from the west, but its true meaning is uncertain.
For セイ, imagine saying "say" to a rare grain: "say" -> セイ, as if asking it to reveal its name.