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Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs