Used to describe an action to overcome a strong feeling or desire by fully experiencing it. She went bungee jumping to finally get her fear of heights out of her system.
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Colloquial Usage
This phrase is often used in casual conversation, not in formal contexts. I just needed to scream to get my frustration out of my system.
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Metaphorical Sense
It does not imply removing a physical substance but refers to a figurative release. He painted all weekend to get his creative block out of his system.