tallow tree
桕 refers specifically to the tallow tree, a tree whose seeds are used to produce vegetable tallow.
桕 is a phono-semantic compound: 木 (tree) provides the meaning, and 臼 likely contributes the sound. The character was created specifically for the tallow tree.
A tree (木) with a mortar-like shape (臼) on the right: imagine the tree's seeds being crushed in a mortar to extract tallow.
For キュウ, picture a tallow tree shaped like the letter Q: Q -> キュウ, and the tree's seeds are used to make tallow.