You're going to have after-work drinks with some coworkers. One coworker doesn't usually come. Today he seems stressed. You want to invite him to come with you, so you say:
Come on out with us and blow off some steam!
"Come on ___" means "come" but sounds more casual and friendly. Here are some examples:
We're having a little get-together at our place. Come on over if you're not doing anything.
Come on in.
Come on up to my office whenever you're ready.
"Going out with" someone usually means going to a restaurant or bar with them:
Use "go" when you and the listener are going to go to the place together.
Use "come" when you're already at the place, or when you already have plans to go to the place with other people and aren't sure whether the listener is going to come with you.
Another meaning of "going out with (someone)" is going on a date together.
To "blow off steam" means to get rid of stress. You "blow off steam" by doing things like drinking, complaining about work with coworkers, yelling at someone, or maybe doing exercise or playing video games.
This expression comes from the old days of steam-powered engines. When the pressure inside the engine got too high, the engine operators had to let some of the steam out. Stress is like steam that has to be released.
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