expression, noun
big but useless person; all show and no substance
Idiomatic expression comparing a person to a large udo plant, which looks impressive but is too soft and inedible. Used to describe someone who is physically big or holds a high position but lacks ability or substance.
彼は体は大きいが、ウドの大木で何の役にも立たない。
He's big, but he's just a big useless lump, good for nothing.
あの部長は独活の大木だ。
That department head is all show and no substance.
From 独活 (udo, 'Japanese spikenard plant') + の (possessive particle) + 大木 (large tree). The udo plant grows tall and looks like a substantial tree, but its stem is soft and hollow, making it useless as timber. The phrase is used metaphorically for a person who is big or high-ranking but incompetent.