Diana: This week was so stressful! I need to blow off some steam this weekend...!
Anna: I hear you~! How 'bout we get together with some friends tonight?
Diana: Yes!! I'm ready to go dancing!!
Anna: Okay, let's all meet at that new club downtown.
Mom on phone to friend: When is the rain going to stop? My kids are driving me crazy!!
Friend: Sounds like they need to blow off some steam! Don't worry, it's supposed to be sunny tomorrow.
Mom: Thank goodness. They've been inside for 3 days straight. They're pent-up with energy and need to get out and run around.
Meaning: to release one's extra energy or emotions.
This idiom probably comes from the fact that boiling water produces steam which creates high pressure that will cause an explosion if it isn't released somehow.
Note: "to blow off steam" and "to let off steam" have the same meaning.
In Example 1, Diana wants to go dancing to let go of her stress.
In Example 2, the children have too much energy because they haven't been able to go outside and play. They need to go outside and release their energy.
The idiom "to blow off steam" was taken from Unit 2 (It's Janet's Birthday!) in LSI's textbook Speaking Transitions for Level 4 Speaking/Listening classes.