let off steam, to

let off (some) steam

 and blow off (some) steam 
1. Lit. [for something] to release steam. The locomotive let off some steam after it came to a halt. With a great hiss, it let off steam and frightened the children.
2. Fig. to work or play off excess energy. Those boys need to get out and let off some steam. Go out and let off steam!
3. Fig. to release one's pent-up emotions, such as anger, usually verbally. I'm sorry I yelled at you. I guess I needed to let off some steam. She's not that mad. She's just letting off steam.
See also: let, off, steam
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

let off steam

mainly BRITISH or

blow off steam

mainly AMERICAN
COMMON If you let off steam or blow off steam, you do or say something which helps you to express or get rid of strong feelings of anger about something. Note: The following expressions refer to the use of steam to provide power for a machine, especially a steam engine. Our teams meets every two weeks, giving everyone a chance to let off steam. I was so annoyed, I had to go for a run just to let off steam. The discussions offer students an opportunity to blow off steam about their teachers. Note: The reference here is to steam escaping noisily from the safety valve of a steam engine.
See also: let, off, steam
Collins COBUILD Idioms Dictionary, 3rd ed.

let (or blow) off steam

get rid of pent-up energy or emotion. informal
The image here is of the release of excess steam from a steam engine through a valve.
See also: let, off, steam
Farlex Partner Idioms Dictionary

ˌlet off ˈsteam

(informal) release energy, strong feelings, nervous tension, etc. by intense physical activity or noisy behaviour: He lets off steam by going to the gym after work. All children need to let off steam from time to time.
See also: let, off, steam
Farlex Partner Idioms Dictionary

let off steam

verb
See blow off some steam
See also: let, off, steam
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

let off steam, to

To give vent to one’s feelings, or to work off excess energy. The term comes from the safety valve in steam locomotives, which prevented steam from building up to the point of exploding. Henry James used it in a letter in 1869, “I feel an irresistible need to let off steam periodically and confide to a sympathetic ear.” See also blow off steam.
See also: let, off
The Dictionary of Clichés by Christine Ammer
See also:
  • let off
  • let up
  • let up (on someone or something)
  • let
  • let (someone or something) get out
  • let go of
  • let us (do something)
  • Let us do
  • let (someone or something) get out of (something)
  • whip back