/ kəˈnjuːt; NAmE -ˈnuːt/ noun
used to describe a person who tries to stop sth from happening but will never succeed :
His efforts to stem the tide of violent crime have been as effective as Canute's.
ORIGIN: From the story of a Danish king of England who was said to have stood in front of the sea and shown people that he was not able to order the water that was moving in towards the land to turn back. The story is often changed to suggest that Canute really thought that he could turn back the sea.
For more information see the Cultural Guide