You could've fooled me!

[wowenglish1]

What does she mean by the next sentence?
You could've fooled me!


A: Sorry about the cake, but I'm only human!
B: You could've fooled me!

 

[Chicken Sandwich]

NOT A TEACHER

3 you could have fooled me spoken used to show that you do not believe what someone has told you : ‘Look, we’re doing our best to fix it.’ ‘Well, you could have fooled me.’

fool - Definition from Longman English Dictionary Online

 

[Raymott]

Well, the wife (presumably) is not disbelieving that he's a human.
She's saying, "You're doing a good job of behaving like an animal, so I could have almost been fooled that you weren't human."