It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].

Today’s kotowaza

うどの大木　Udo no Taiboku

Few words for you.

It is tasty when mixed with sugar and vinegar.

Daiboku – a big tree

Good for nothing (of large people); big but useless

Its origin

When a Udo grows it becomes about 2 meters but its stem is soft and useless.

Its meaning

A person who has a big body and great but is useless people. We use when we talk about someone with false impression, deceptive appearance.  

How to use it

弟は人一倍、体が大きいのに、体を動かすのが大嫌い。母にお使いを頼まれても、何やかんや言っていかない。「まるで、ウドの大木よ。」と母に言われた。

Otouto ha hito ichibai karada ga ookii noni, karada o ugokasuno ga dai kirai. Haha ni otukai o tanomaretemo, nannya kanya itte ikanai. 「Marude, udo no taiboku yo 」to haha ni iwareta.

My bother is bigger than others. But he hates moving his body. Even if his mother asks him to do shopping, he always find a way to make excuses and does not go. 「As if he is udo no taiboku isn’t he. 」my mother said.

arigato