Fig.: to relieve stress, to release excess energy or anger through some kind of activity

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 and a T-shirt

È un vero stakanovista con delle priorità un po' confuse. Quando ha bisogno di sfogarsi, viene a lavorare di sabato in blue jeans e T-shirt

Mother was very worried because young Bobby was really late coming home, and, finally, she blew off her steam fighting with innocent father

La mamma era preoccupatissima perché era veramente tardi e  Roby non era ancora rientrato, così, alla fine, si è sfogata litigando con l'incolpevole marito

Don't worry at Lucy's shouting at you. She's not angry at you, she's just letting off some steam

Non fare caso se Lucy ha alzato la voce con te. Non è arrabbiata con te, sta solo scaricando la tensione accumulata

"Where's Tom?" "He had a bad argument with his girlfriend, so he went for a run into the woods to let off steam"

"Dov'è Tom?" "Ha litigato di brutto con la sua ragazza ed è uscito a fare una corsa nel bosco per sfogarsi"

After a frustrating 9 1/2-hour session with the Unions, the executive went running to let off steam, rather than meet the press and act testy with reporters

Dopo un frustrante incontro di 9 ore e mezza con i Sindacati, piuttosto che incontrare la stampa e trattare stizzosamente i giornalisti, il dirigente ha preferito andare a correre per scaricarsi un po'

In a boiler, the heated water turns to steam, which can create considerable pressure in the container. If the pressure becomes too great, there is a danger of explosion. For this reason, the boilers are equipped with safety valves, called “blow off valves”, that open up and relieve the pressure by letting the excess steam out. So, “blowing off steam” means “preventing explosions” in a literal, as well as figurative way.
The figurative use of this expression dates from the early 19th century, referring to real steam engines, especially of locomotives.