be a monkey's uncle

/biː ɐ mˈʌnkɪz ˈʌŋkəl/

be a monkey's uncle

to display complete surprise or disbelief

Examples

1. If he was over thirty, then I was a monkey's uncle.

2. I may agree that if two plus two equals five, then I am a monkey's uncle.

3. And I will be a monkey's uncle if I have you ruin this for me.