Meaning: To enjoy oneself by relaxing normal formalities.
Example: He is a true workaholic who has misguided priorities, when he wants to blow off some steam he comes to work on Saturday wearing blue jeans.
Origin: Boilers are commonly used in steam heating systems and steam engines such as those used in a steam locomotive. The boilers contain water that is heated by burning some fuel such as oil. The heated water turns to steam, which is then sent through a system of radiators (in the case of heating systems) or harnessed by a steam engine.
The steam creates considerable pressure in the boiler. If the pressure becomes too great, there is a danger of the boiler exploding. Hence boilers are equipped with safety valves called blow off valves that open if the pressure becomes to great.
"Blowing off steam" prevents explosions by relieving the pressure in a boiler by venting excess steam and pressure.