Definition
This phrase is used to express disbelief in what someone has just said, often implying that the person's words or actions do not align with their true feelings or intentions.
"I don't care at all, really." "Oh really? You could have fooled me!"
"I'm not disappointed with the results at all." "Well, you could have fooled me, I don't think so."
"To be honest, I actually liked that restaurant." "Really? You could have fooled me, based on your expression earlier."