If someone throws a spanner in the works, they prevent something happening smoothly in the way that it was planned, by causing a problem or difficulty.

What does it mean to throw a spanner?

Definition of put/throw a spanner in the works

informal. : to cause something to not go as planned We were ready to start the project when the bank threw a spanner in the works by denying the loan.

Where did the saying throw a spanner in the works come from?

Legend has it that Mr Ewen McGregor set up the derrick and drilled a hole 700 feet deep. All was going well and signs of an oil strike were imminent when his son Jethro dropped a spanner down the hole. From this incident the term, 'A spanner in the works', originated, and is now known world-wide.