you could have fooled me!

you could have fooled me!

used to express cynicism or doubt about an assertion. informal
See also: could, fool, have

you could have ˌfooled ˈme!

(informal) used for expressing your surprise about a statement, claim, etc: ‘He’s quite intelligent, you know.’ ‘You could have fooled me! I’ve never heard him say anything intelligent at all.’
See also: could, fool, have
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