What it means
Means to mess things up or derail a plan just when it was all running sweet. Picture literally chucking a spanner into a bit of machinery so everything clunks to a halt. You can use it for a tiny hiccup or a full-on disaster, especially when one daft detail sends the whole day sideways.
Usage examples
"We’d planned a tidy BBQ, then Gaz rocked up, kiddie pool in tow, and the council bloke showed. Proper spanner in the works, burgers under a brolly."
"Everything was booked, then the airline strike threw a spanner in the works."
"Don't throw a spanner in the works now, we've nearly got the deal signed."
"We were all set for a clean little launch, then the payment system died ten minutes before go time. Absolute spanner in the works."
"Mum said yes to the trip, then my brother got suspended and that chucked a proper spanner in the works."
Where it comes from
This one comes straight out of British workshop imagery. A spanner is a wrench, and the works are the moving parts of a machine. If you lob one into the mechanism, the whole thing jams or breaks. By the late 19th century, people were using it figuratively for anything that suddenly wrecks a plan.
