(To) Blow off steam

In reality a person cannot blow off steam (the hot rising air from boiling water) — only electrical equipment can, such as the electric jug (appliance for boiling water for coffee). So what does it mean when a person blows off steam?

If you’re feeling angry, stressed or are experiencing some strong feelings and you want to get rid of them so you feel better again, you will blow off steam by doing something such as exercise to get rid of the stress.

“Why is Nick so angry and where did he go?”

“He had a fight with his brother, so he went for a run to blow off his steam.”