An idiom meaning someone or something that looks impressive or important but is useless or ineffective.
部長は肩書は立派だが、実際には何もしないウドの大木だと言われている。
People say the department head has a grand title but does nothing in practice — a 'udon no taiboku' (big but useless).
見た目は豪華だが、使い道のない施設はウドの大木だ。
A facility that looks luxurious but is useless is a 'big but useless' (udon no taiboku).
彼は会議でいつも偉そうに話すが、結局何もしないウドの大木だ。
He always speaks grandly at meetings but ends up doing nothing — a big but useless person.