(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