trifoliate orange tree, thorny tree used for hedges
枳 refers to a specific thorny citrus tree, the trifoliate orange, often used for hedges. Its meaning is narrow and botanical, centered on this particular plant.
枳 combines 木 (tree) with 只, which likely contributes a phonetic element. The character specifically names the trifoliate orange tree.
A tree 木 with a distinctive right side 只: picture a thorny hedge of trifoliate orange trees, their branches forming a barrier.
For キ, imagine a key-shaped thorn on the trifoliate orange tree: key -> キ, and the sharp thorn locks the hedge tight.