A chaotic, poorly designed, or badly assembled project that barely functions. The phrase evokes the clanking, uncoordinated sound of a sack full of loose tools. Workers use this term to describe inherited messes, badly written software, or mechanical repairs that are guaranteed to fail and cause endless headaches for the next person who touches them.
Heard on the job
I opened up the housing and the wiring in there's an absolute bag of spanners.
Insider tip
If a veteran mechanic or developer calls your work a bag of spanners, you need to tear it down and start over.