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general::
Phrase(s): (You) could have fooled me.
Inf. I would have thought otherwise.; I would have thought the opposite. • Henry: Did you know that this land is among the most productive in the entire state? Jane: You could have fooled me. It looks quite barren. • John: I really do like Mary. Andy: Could have fooled me. You treat her rather badly sometimes.
McGrawhill's American Idioms And Phrasal Verbs