To confuse or surprise someone so much that they are not sure how to react or what to think.
From Latin *confundere*, meaning "to pour together, to mix up" (*con-* "together" + *fundere* "to pour"). The idea of mixing things so they can’t be told apart led to the sense of confusion.
When your brain is confounded, it’s like someone poured all your mental files into one messy pile. Statisticians still use "confounding" for variables that get mixed up and trick us into seeing patterns that aren’t really there.
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