This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
櫤 combines 木 (tree/wood) with 箭 (arrow). The exact historical development is uncertain, but it likely relates to a type of wood or a wooden arrow.
With 木 (tree) and 箭 (arrow), picture a wooden arrow or a tree used for making arrows. The meaning is unclear, so treat it as a wood-related object.
For セン, imagine a thousand (千, せん) arrows made from this special wood: 千 -> セン, and the tree provides the material.