Perform or portray something or someone, as in As she read to the class, the teacher had each child act out a different character in the story. [c. 1600]
Express unconscious feelings or impulses through one's behavior, without being aware of it. For example, She acted out her anger at her father by screaming at her husband. This meaning comes from 20th-century psychological theory and usually (but not always) refers to negative or hostile impulses and emotions. The term is sometimes used without an object to mean "misbehave" or "behave disruptively," as in The child is acting out in class. [First half of 1900s] In both usages, out means "openly" or "publicly."
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