Une usine à gaz can be translated as a complicated way of working or a labyrinthine system.
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Translated literally, une usine à gaz means a gas plant or factory.
This expression dates to the French industrial revolution in the 19th century.
During this period in France, gas factories started being built in industrial zones.
Typically they were composed of complicated systems of pipelines and crank handles that were incredibly difficult to understand.
You can imagine a maze of pipes twisting and turning.
Hence the use of une usine à gaz to describe any incomprehensible system.
More specifically, it can be used to criticise a system that is big, expensive and complicated, but ultimately does not serve much purpose.
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