(informal) a variation of spanner (or wrench) in the works, a person or thing that ruins or causes confusion to another person's plans, work, etc. Dog is a name given to various mechanical devices, usually having or consisting of a tooth or claw, used for gripping or holding. The expression comes from the fact that such a tool might badly damage a machine if it were thrown into it while the machine was in operation. —PDC Volume 4 Approved Glossary