(nineteenth century engraving)
Then King Canute explained to his courtiers that nature would not obey even a king as powerful as he was.
This story is probably not true, but it is very old and goes back to the twelfth century. Later versions of the story say that King Cnut was not stupid to order the sea to stop, but was showing his followers the limits of kingship and of human ability to command nature. So he was making a wise point.
So how does this story find its way into modern English? Actually, you sometimes find people making an allusion to this story in conversation or in writing. They might say that somebody is like King Canute. Or that, like a modern day King Cnut, somebody is trying to hold back the tide and the waves.
Usually, this means that somebody is making a futile attempt to hold back something unstoppable; the idea that the king was really a wise philosopher is often lost.
So if you see a reference to King Canute and waves or the tide, then you will know that it involves trying in vain to stop something more powerful than you are.
This kind of reference is not exactly an idiom, as the words used may differ a bit each time. It is used as an analogy.
The story is memorable, and popular to illustrate too. You may find it in a modern political cartoon, and books like to depict it too.
https://commons.wikimedia.org/w/index.php?curid=35011826
This story is the only thing that many people know about King Cnut, and it is probably not true! When something is well known, but not true, we call it apocryphal. But it is a very interesting idea that continues in modern culture.
Vocabulary:
courtier - someone at a royal court
kingship - the position of being a king
analogy - a comparison of two things which are like each other in some way; here, a modern day person will be compared to King Cnut
apocryphal - well-known but probably untrue and did not happen
futile - useless or hopeless