Let off steam
Means: To release extra energy or emotion.
That's what people do when they need to get rid of negative, or bad feelings. This phrase comes from steam engines, when pressure builds up inside, the engines must release steam.
Examples:
After the exam, the students went out together to let off steam.
Bob has a very stressful job. After work, he likes to go to the gym to let off steam.
#idiom
