‘To act casual’ is an expression we use to express that we are pretending to be unaware of something (often something that we ourselves have done) when in reality we are fully aware of what has happened. For example:
Mother, “Who ate all of the cookies that I made for the church fete?”
Little Billy, “Joey, what shall we tell her?”
Little Joey, “We won’t tell her anything, we don’t know anything about the cookies, just act casual!“
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