Here's how it developed:
* Steam Engines: Steam engines worked by generating steam pressure, which drove pistons and performed work.
* Pressure Relief: If the steam pressure built up too much, it could cause a dangerous explosion. To prevent this, steam engines were equipped with safety valves. These valves would release excess steam, reducing pressure and preventing damage.
* Metaphorical Extension: The act of releasing steam from a pressure-filled boiler became a metaphor for releasing pent-up emotions or stress. The idiom "blowing off steam" emerged to describe any activity that helped alleviate emotional tension, much like the safety valve released pressure in a steam engine.
Therefore, the idiom "blowing off steam" originated from the practical function of steam engines and their safety valves. It's a metaphor that captures the idea of releasing built-up pressure, whether it's steam in a boiler or emotions in a person.