This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 榀 is uncertain. It visually combines 木 (tree/wood) with 品 (goods/article), but the original semantic or phonetic motivation is unclear.
The kanji has 木 (tree) beside 品 (goods). Picture wooden goods stacked in three tiers, but remember that the actual meaning is uncertain.
For こまい, imagine a small wooden frame (木) holding three delicate items (品) — a 'koma' (こま, spinning top) and an 'i' (い, a single thread) tied around them: こま + い -> こまい.
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