blow off steam

Example: “After a long week, I blow off steam at the boxing gym.”

Origin: Carries more menace in its original setting than the modern casual use suggests. A nineteenth-century steam boiler under pressure was a genuinely dangerous object, and boiler explosions were a routine industrial and maritime catastrophe — American riverboat accidents alone killed people in the thousands over the century. The safety valve existed to vent excess pressure deliberately before it could build to the point of rupture, which made letting off steam not a matter of comfort but the specific procedure standing between a working engine and a lethal failure. Figurative use is attested from the 1830s and 1840s, when steam power was new enough for the mechanism to be widely understood, and it carries the same implication: the pressure is real, it is going somewhere, and controlled release is preferable to the alternative.