hollow in a tree; tree hollow
Refers to a natural cavity or hollow inside a tree trunk or branch.
In the forest, an owl lived in the hollow of an old tree.
Compound of 木 (ki, 'tree') + の (no, possessive particle) + 虚ろ (utsuro, 'hollow, cavity'). The word 虚ろ itself is of native Japanese origin, with the exact derivation uncertain.