oak for charcoal
櫟 refers to a specific type of oak tree, particularly one used for making charcoal. Its meaning is narrow and concrete, centered on this tree species.
櫟 combines 木 (tree) with 樂, which likely contributes the sound and may relate to the tree's use or appearance. The exact historical development is uncertain.
The tree radical 木 on the left marks this as a type of tree. The right side 樂 suggests music or enjoyment, but here picture a specific oak tree whose wood is prized for making high-quality charcoal, perhaps for traditional ceremonies or enjoyment.
For レキ, imagine a lucky oak tree that always produces the best charcoal: lucky -> レキ, and the tree's luck makes it the top choice for charcoal.